Something I've always noticed, at least in most WH40k lore and art, is that the more experienced the soldier, the more decorated they are, as in reality. It would be fitting for players to earn purity seals and scrolls or badges, even Iron Halos and laurels, to place on their armor after accomplishing certain feats and for faction-specific/legion-specific reasons. I'll stick to purity seals here as a basic example, but there are so many different types of badges and totems in WH40k lore that the possibilities are practically endless. (For a full list: http://warhammer40k.wikia.com/wiki/Honour_Badges_(Space_Marines))

Not just for purely cosmetic purposes, they could be used like wargear, for defensive or offensive enhancements written into the seals. A highly-decorated Space Marine would be even more dangerous, not only because of his merit and skill, but due to his numerous purity seals.

Specifically, I think the best option for actually earning the purity seals would be exactly how the actual Space Marines earn them in lore. By accepting incredibly challenging (at least for the individual player) assignments or missions, their chaplains would create them to be worn and if the Marine comes back alive, the seal remains as a permanent symbol to be worn on his armor or weapons as a testament to his faith and purity. In the case of the game, maybe if you fail the assignment, you have to wait a long amount of time (days) before you're allowed to undertake it again.

As far as functionality goes, these would mostly be personal achievements to add some extra visual flair and customization to your armor and weapons, as well as increase certain stats like defensive or offensive properties of the piece of gear you place them on. For example, you could earn a purity seal and place it on your breastplate to increase the defense of your power armour, and earn another one to place on your Bolter that increases its rate of fire or makes it slightly stronger against specific types of xenos. Then so on for each piece of wargear, to the point where you can have near-unlimited numbers of combinations of upgrades, only limited by your number of purity seals.

For more information on purity seals lore, the Lexicanum is a good place to start: http://wh40k.lexicanum.com/wiki/Purity_seal#.UpRhOeJGklA

Other factions would earn their own specific types of upgrades for the same purpose (stat bonuses and cosmetics), possibly things like Marks of Chaos, Collars of Khorne, and the like.

Now, with all the possibilities and whatnot on the table, could this possible be implemented in the game? And would it seem like something the community would want?

Something like this could easily get out of ways in multiple ways.

First, adding a lot of small stat bonuses for players to hunt isn't great for the overall structure of the game. Everyone's goal should be the same: contribute to the war effort. All of your rewards should come from this. Making players stronger (even very slightly) for gaining achievements will result in a great deal of players that go their own way. For example, in TF2, when Valve first added new unlockable weapons tied to achievements with the Heavy update, one of the achievements was to eat 100 sandviches. Every server had groups of players standing at the spawn eating sandvich after sandvich instead of actually trying to win. Updates today no longer tie weapon unlocks to achievements.

There's also the risk that a player with a lot of these bonuses winds up notably stronger than one without.

Finally, tying visual customization to stats isn't a great idea. The last thing we want is for everyone to look the same because of the bonuses they want. Flavor of the month builds will have that effect.

When you see a decorated Space Marine coming your way, you know he's going to be a dangerous enemy not because he has more stat bonuses than you, but because he earned those decorations and he's just really good at the game. Hats in TF2 provide no statistical benefit whatsoever, but when I see I guy wearing a rare hat, I figure he's probably been playing a long time. More often than not, it's the people with the flashiest hats that top the scoreboard.

The only confirmed thing so far is:
All the players that participated in a campaign will receive a medal to put on their armor.
(source thread:http://forum.eternalcrusade.com/threads/compilation-of-info-about-eternal-crusade.1959/)
Currently they are planned as cosmetics that don't give any extra bonuses.
As for race medals/seals SM go for purity seals , medals, halos etc (as you stated). CSM usually go for armor decorations(spikes, skulls , tubes etc.) , sometimes armor corruption , limb replacement etc. Orks - war throphies , spikes , iron plating , extra teeth for that ork specific grin.
The issue would be eldar. Some of their classes have very simple attire and carry nothing but their gun,ammo and soulstones. Perhaps small pennants of some sort.

Perhaps I should have clarified better, but considering that this is a "90% pvp MMO" per what we've heard from the dev team, I don't think anyone is going to be able to just "go their own way" easily and earn these things. These achievements are NOT meant to be PvE earned. They are meant to be difficult and only achievable through defeating enemies of opposing player factions. The only way Space Marines or CSMs in the lore earned these types of decorations is through missions that would surely end with their deaths, more often than not, if they were anything lesser. The idea is for these to be teamwork-based achievements to strengthen the individuals playstyles and make teams/squads stronger as a whole, while allowing each player to customize things to his or her own style.

You also assume that I meant for them to be all "cookie-cutter" items, when that's not the case (that result in aforementioned FotM "builds"). As I said in my original post, these will be sort of all-purpose upgrades with the intention being that each person will choose a different place to place his seals. If player A feels like he gets into bad situations a lot and takes too much damage or he takes a more frontal role in combat than others in his squad, he might opt to place a purity seal on his chest (given my previous example), to better ward off certain types of damage (i.e., a blessing to weaken the enemy's melee strokes). However, player B might choose the offensive route and place a seal on his Bolter to increase the rate of fire or expand magazine capacity if his role is more of a backline situation where he doesn't have to worry about much damage, but this could be a trade off with reduced accuracy and maybe slower time to aim due to a heavier weapon.

I don't suppose in any way that this game is meant to be gear-based, as stated by the devs, it's mostly horizontal progression with balanced weapons all suited for different purposes. So, with these types of upgrades, it will allow for minor vertical progression to make people who invest their time, effort, and money into the game actually feel as if they've earned something. A superior player will gain superior tools to aid himself in many ways, not necessarily through direct statistic superiority but through customization options that will fit his playstyle that can impact certain things both positively and negatively, as mentioned above.

Think of it akin to being able to place attachments on your weapons in a first-person shooter. They don't really give you any sort of true advantage over anyone else that already has them, but they help you play better if you use them to their best ability. Except here, we have the double advantage of looking bad ass at the same time.

Another thing I forgot to mention: possible squad decorations or banners and standards that can be equipped as well, more as a prestige thing than anything. That would be interesting, to know when you're facing down an experienced squad as a whole working together.

However, I do agree with you that these should not be as typical MMO fare gear, like an endgame set that everyone wears and looks exactly the same with, or BiS gear that makes everyone identical. When I suggested it, I intended for each upgrade to be tailored for the specific player that wears it, based on allegiances, performances, his roles, and various other factors.

CSM would more than likely gain Marks of <insert Chaos deity here> and mutations, as well as the spikes and skulls you mentioned. From a lore standpoint Chaos Marines wear certain types of glyphs and marks similar to seals of purity that are essentially the opposite, they inflict negative effects on their enemies around them rather than protecting their wearers, such as confusing them or attacking their minds. I'm not sure if that would be a good thing to implement into the game, however.

Orks would obviously just get more dakka.

Eldar, I'm not typically well-versed in their lore so I wouldn't know. I'd assume maybe (enchanted) tattoos or engravings would be more their speed as opposed to directly upgrading their physical armament. They don't bare much heraldry into battle as far as I know either, so it would be a bit of a toss up thing.

A major problem that stands out for me is that your example doesnt allow for a limit. What if just in the first 2 Campaigns a player is particularly good, gets the 2 Campaign seals and completed 5 challanges during the Campaigns. Does that mean my Character now has 7 Purity Seals just within the first 6 months of release? Does this number keep going up? Not only do i start to look stupid, but if each one conferrs some kind of bonus now im damm well invincable.
This starts to sound like too much as a upgade and too much like making good players even better. If you have a limit on the # of them, and they have a very low effect then im okay with it. Adds to the customization feel and as they should be particularly hard to aquire creates that Vetren look to them.
Why cant they be PvE earned? Are 'nids not a major threat both in the fluff and to your ability to control and hold terrority and land in Eternal Crusade? Why shouldnt i be recgonized for taking the less glorious road of PvE when it still has a tangiable benifit to my Factions war effort?
I'm just not sure, there are a few holes in your argument. I'm all for customization and rewarding some players, but not to the effect that it starts to change their stats.
Adding in Banners is a very cool idea, when Squad level ones say on the Sergent (behind the head on a pole type deal) but not when they have any effect. The last thing we need is DA-like Dakka banners everywhere.
Sorry for the overall negative opinion, i'm just not a fan of anything that starts to feel like equiping stat incressing gear.

Keep in mind that I'm describing not the flaws with your suggestion but the potential flaws. I know you wouldn't want it to happen as I described, but you also underestimate the value players place in tiny vertical upgrades. I don't know what sort of achievements you have in mind for earning these, but with such a system, it's very easy for a single designer in a moment of poor judgement to create an achievement that results in groups of players doing things that don't actually help them win, such as using weapons unsuited to the situation or facing more enemies than they can handle and hoping for the best. It's not that this will happen if such a system is in place. It's that in designing systems, you want to make them unbreakable.

Remember, that superior player you mention will be the last one to need superior tools. If you're good at the game and you look awesome, you'll feel plenty like your time and effort has payed off.

And I can't really state it enough. Tying stats to visuals is something even the most gear-based games are moving away from. People like to look the way they think is cool, not the way whichever dev assigned to stat the gear thought was cool. If I absolutely love the bonus from the iron halo, I don't want to force myself to wear it if I think it's ugly and distracting.

Considering all anyone can do is poke holes in issues that aren't really there until they've made them, then sure, I guess there could be holes. I would say I'd leave everything up to the devs as far as a limit goes and exactly how much of an upgrade they are. If you noticed in my previous posts, I never said how much or how many. I only said they could be available. It would not be my choice to make as far as those things go, because I don't know exactly (nor does anyone but the devs) how the game is balanced right now and how things like this could effect it.

If it were up to me, there would be a limit to the number you can actually equip on each item, with various cosmetic choices and options (studs instead of seals on your armor and things of that nature, not strictly purity seals) available for placing. No, you can't place them in weird or ridiculous/uncanon places, since I know someone is going to ask. And no, I'm not suggesting they all look completely and exactly the same even though.

The actual upgrades would be minimal, mostly in terms of the defensive things as "attachment" type upgrades would be functionality additions and not statistical. If you know the lore or read the links I posted, you'd know that most of the actual things they do are more of a placebo effect to make Space Marines more brave in combat than any true beneficial qualities.

The upgrades in question (that might throw the game out of balance against other players, the glaring issue people keep pointing out), will only be earned through PvP because PvE will be incredibly limited in quantity to begin with, and might be much easier to accomplish and "farm" them for people who aren't actually skilled. There is room for PvE-specific upgrades, however, but I won't speculate on that.

I will say again, this is not intended to be seen as a "gear" option, but as playstyle customization unique to each person to better suit their needs and shape their role.

Of course, I appreciate the sight into things I've not thought of, but I'm not a developer and if it were to be used I'd leave it in their capable hands. I'm merely outlining a basis of things that I think would be interesting and add to the experience of the game and growing your character in the world from an initiate to a hardened veteran.

As far as visuals and cosmetics, I don't mean for them to be rewarded as items and gear are in MMOs. I think people are getting the idea that they would be given like quest rewards, a "Choose one: Sword of Positive Effect 1 or Shield of Positive Effect 2" sort of deal. The reward you receive will be tailored to the need that you had for it and depending on what part of your gear you want it to go, it could be multiple things.

Say you want an armor upgrade and you're a Space Marine, you'll have a choice of purity seals, studs, iron skulls, aquilas, laurels, any type of canon heraldry your particular faction and legion would typically use. One marine could have earned three of his upgrades and have an iron skull (of a chosen color) on the chest of his unique armor, studded knee plates, and an iron halo, and another could have two purity seals on his shoulders and a laurel. Or, maybe you had a different set of armor for a different purpose and it has completely different decorations, to indicate the trials its been through and also to serve you better for that role.

I might even go as far to say that armor could be dented or scarred for failed assignments or particularly bloody ones, to add an even deeper element of immersion, but I won't go there.

Hmm fair enough.
I would almost prefer this to be a reward for accomplishing specific actions. Now this is the same thing as you said i know, but im thinking more along the lines of Space Marine with their variant of "killstreaks". So for example you make 500 head shot kills with your bolter and you unlock a cusomization like a purity seal that goes on your bolter. You make 1000 and you get something different. This doesnt confer a bonus but still acts as a reward and is a recgonizable feature to make your acheivement stand out. The idea being that by the time a player reaches Veteran status that they look the part.
With this you dont have to limit a player expect for in what they skill level allows them to unlock vs time.

What kind of "attachment" tyoe upgrades are you thinking that wouldnt overly effect balance??
Why would PvE be incredibly limited to begin with? Other then the Dev's letting players establish themselves to begin with and not sending 'nid waves out immeidately they should still have a firm presence in the game, just not in heavy PvP areas.