Now you're thinking more along my lines.

They wouldn't be so much as "assignments", but more like things that will happen fluidly over the time you play, both cumulative upgrades earned with "experience" with weapons or killing specific enemy types and things that can be earned based on performance in a given time. And so when you come out of a particularly nasty and strenuous firefight that may have been impromptu or occurred in dire circumstances, you can not only say, "That was ridiculous", but you also get a medal slapped on your chest because everyone saw you and your four remaining squadmates wreck an entire battalion of Ork Boyz like a train running over a broke down jalopy because of your superior tactics.

Maybe based on something like, "Get 100 enemy player squad kills in under 5 minutes", something nearly absurd and seemingly impossible but that can be accomplished at the right place, right time, and right amount of skill. This way both the average player can progress through simply playing the game and skilled players will stand out among them with slight tweaks at their disposable.

The "attachments" bit wasn't necessarily about actual attachments, but more as upgrades that function similarly. I imagine we'll have real and actual attachments for our weapons, and these upgrades would act as a sort of supplement for them, as well as being able to perform marginally on their own. Think of it like this: if you use a particular weapon for hours and hours, you'll eventually become skilled with that weapon and be able to reload it faster, manage the recoil better, and various other things. They will be there to emulate things like muscle memory in real life. Through using the weapons, you'll unlock upgrades that will be weaker than actual attachments, but can provide similar benefits. Don't get me wrong, they will not be able to serve as substitutes for attachments themselves. And if someone has a better attachment for a given situation than you, they will outperform you on a statistical basis. These serve merely to enhance or allow greater customization of weapons on a broader scale.

And on the PvE subject, I've really not looked much into it, but going on the dev statement that most of the game will be PvP, I don't think it would impact things too much to have separate upgrade types.

I guess it's worth pointing out that I'm okay with perks in general. The small bonuses you're describing are essentially that.

What I'm not okay with are two things: tying them to very specific achievements and giving them disjointed visual representation. Rewarding a player with a perk for killing 100 enemies in melee is fine. Rewarding a player with a perk for killing a meganob with a combat knife is not. An accuracy bonus shown as a sight on your gun is fine. An accuracy bonus shown as an iron halo is not.

It sounds like you're suggesting that upon receiving a specific upgrade/perk, you can choose how it will be visually represented. Thus the iron halo doesn't indicate that a player has Upgrade X but that the player has some upgrade. If that's the case... what's the point? Why not just let me unlock perks and cosmetics separately and equip them in whatever fashion I want?

The rewards, at least in my own idea, should be based on statistical things and performance, like kill/death ratio in a given time or situation or killing X enemy type 500 times. And they cannot be repeated, once you accomplish that particular thing with that particular weapon, you can't do it again. There may be more room for growth past the upgrades in some form of another, but I really can't think of what that might be, besides squad-based perks.

That's not what I'm suggesting at all. As I said in my original post, the visual representation will be based on what you upgraded, not how. If you upgrade your armor, the cosmetic will be something on your chest, shoulders, or legs. If you upgrade your accuracy on your gun, it may be represented as a purity seal on your Bolter (known to have written incantations to improve accuracy or make a weapon more effective against a given enemy type, through a placebo effect on the Space Marine or an actual blessing isn't really clear). If you upgrade some sort of HUD element or an optic for enhancing your zoom capability, it will be shown as a monocle on your helmet or implant on your face (for example).

I'm of the opinion that exceptional achievements should unlock rare and visually distinctive cosmetic options.. but not give any game benefits.

The combination of these two points are already so well-said, that I don't think I can say it better:

Certain honours can be further distinguished by not merely being "grinded", but requiring an extraordinary (even downright ridiculous) achievement. One example I can think of, would be needing to accomplish a certain number of a certain type of kill, within a time limit.
For example, killing 20 enemies with headshots from greater than 100 meters, with a boltgun, within 1 hour of gameplay time.
Or killing 5 enemies in melee, unassisted (from full health to dead), in the space of 1 minute.

Of course, the specific numbers need to be tweaked based on the intensity of combat we'll actually see in the game, but basically the intention is this:
When you see a teammate you've never played with before with that Marksman's Honours or particular Skull & Motto badge on their armour, you know for a fact that he's got your back.
And when you see an enemy with such an adornment, you can be sure you have mere moments to react, or you're just going to be another notch on his long, long list.

That's why I mentioned something like getting 100 player kills in five minutes between your squad mates. If you have a squad of ten, that effectively means each of you has at least a 10:1 kill/death ratio. And not only that, you carved a crippling swathe of death through multiple enemy squads in less than five minutes of fighting.

And I like that idea, that's the idea I'm intending to convey, but with extra perks for small vertical progression. However, it is probably best left simple, as you put it, and just having these things in game as cosmetics options would be a great thing, in my opinion. They should convey a sense of pride for your teammates and dread for the enemy, possibly even cause them to run. Which is a feeling hard to get across in most games, but it might be possible here.

I don't care much for this. What you're describing is redundant at best and min/max at worst. If I want my reload speed to be as fast as possible, I'd better have both reload speed upgrades. If they don't stack, I've wasted my time getting the perk. Perks and weapon attachments can coexist, but they shouldn't provide the same types of bonuses.

Also, think of it like this: if you use a particular weapon for hours and hours, you'll eventually become skilled with that weapon. As a player. Players have to actually aim their weapons. They have to properly time their reloads. They have to counteract recoil. They will develop their own muscle memory. Perks are fine, but don't get caught in the trap of thinking that our characters won't properly reflect our own personal skill.

Lol good sounds like we are more on the same page now.
I agree with Shroom Mage, if there should be a system like this then i would want certain gear to correspond to certain accomplishments. So for example for me to get that Iron Halo i need to say survive a close to death/down but not out experiance 250 times. Then for those people that really want a particular reward then they can work to meet specific goals to unlock certain gear.
We could also incress the scale of this by having different levels and types of customization. Player level, Squad level, and Strike Force level.
Player Level lets you unlock gear that changes how you look, reference Iron Halo above
Squad Level acheivments lets you unlock gear that has small effects to your play. Such as your example above with reduce recoil or slightly faster reloading
Strike Force Level Acheivments would be more like unlocking banners and the like
Mesh this in with a Class Tree and you would have a ton of reward/acheivement options, particularly when you start to look at class specific ones

Perhaps rather than tying these "perk" type upgrades and achievements to a cosmetic item, they should be just be separate. Originally, I'd have intended that as a way to know the capabilities of your enemy or as a prestige thing, but maybe totally different types of rewards would function better, in two different sorts of achievements.

Functional perks could be earned with accumulated things over time, such as x number of enemy kills, x number of revives/defends, x number of points taken, etc. This would emulate experience gained by your character in things as his performance improves based on what he does the most.

Cosmetic insignia and decorations would be rewarded for "outstanding performance" achievements, such as killing x number of enemies in x amount of time, killing x number of enemies at a distance greater than 150 meters, killing x numbers of enemies in a single chainsword combo, and so on.

This way prestigious rewards maintain their prestige and everyone has access to the functional upgrades to better help them earn decorations as well if they haven't.

I like the idea of the different levels of achievements, those will breed teamwork and allow strike forces and squads to sent their own individual performance goals as well.

I'm going to guarantee this to your guys right now, in Eternal Crusade you will not ever receive a permanent boost over other players via abilities or items that you don't pay for with space in your armor slots.

A more likely scenario if they don't want to limit the amount of medals, rits, and purity seals (or whatever race specific items fit you) on your character that you've earned, you'll probably be allowed to wear all of them, but if they have associated abilities you will only be allowed to activate a certain number of them at any given time. This way you can show how awesome you are, but not have an item advantage over other players.