The main problem I see with literally putting all IoM races in the same faction is that they would be able to use each others' vehicles.

Translation: all the Space Marine clones with OP infantry would be able to use the Imperial Guard's OP vehicles. The Space Marine clones are easy to roll into the main SM faction since they'd basically be reskins with slight stat tweaks, just another chapter really, but the Imperial Guard would need to be independent because giving Space Marines a Leman Russ or Baneblade would be a balance nightmare. Even a Chimera would be over the top if it was carrying Space Marines inside it.

I think people are really underestimating how much vehicles can tilt the odds in the Imperial Guard's favor. Consider that a Chimera can rather easily kill all the other factions' transports, and if you stick an autocannon on there (which is a legit upgrade) it can take on most factions' MBTs.

Further consider that if you just give the players Veterans with carapace armor, they're pretty much equivalent to a Space Marine Scout or Tau Fire Warrior. Which is better than it sounds, it means they'd actually be at an advantage over most standard infantry, Tactical Marines being the sole exception. Even if they are a little weak in the infantry department, their vehicles can easily close the gap. Especially if AV weapons are not spammable.

IG armored vehicles and artillery is their main advantage, of course

Speaking about Astartes using Imperial vehicles and vice versa - this is easily solved
Astartes are almost 3 times bigger than regular human soldiers. Simple answer, don't you think?
SM vehicles are created for Astartes to use and IG vehicles are created for soldiers - this is the reason why they don't use each other's vehicles

In the Warhammer both of them ARE on the same side, they are not different factions. This is the way it should be in the Eternal Crusade if they want to create a genuine feel

But, anyway, we have already discussed why simple infantrymen shouldn't be player-controlled

i love how people keep saying a basic guard men stand no chance against and orc or eldar(space marine and csm maybe) But my lasgun kills them(an there space marine buddy's) just fine thank you very much,(there even more effective in mass) other wise no one would play them table top if they were soo horrible

if you implement guard, i say make the basic IG free to play, the devs have said it once and many time after, the game won't be balanced, it will have asymmetrical balance. Guy one Is not equal to guy Three if Guy one charges him head on. But guy one is greater then Guy three if he attacks from high ground or the flanks.

They're looking to make vehicles multi-crew, so making IG rely on vehicles is a no-go because the game is, last i heard, intended to be infantry based(with some vehicle support) and about player skill.

Again, factions in the tabletop are "equalized", with some difference in tactics and visuals, to make it "balanced" and playable at tournaments. The core mechanic is the same, they just give different designs, looks and story to factions so that people could play whom they like most but enjoy the game just the same as other players playing other factions.

Now, back to the actual roleplaying living world of the Warhammer universe - it's a bit different

Guardsmen are simple soldiers - humans like you and me. They don't have super-regeneration, multiple organs inside, feral bloodthirst or superhuman strength and stamina. Their advantages are the same as in the real world: tactical flexibility, interoperability and numbers. Armor, air, artillery, navy - main holding piers of the Imperial Guard. Without all of this - one soldier is just one ape with a stick, no matter how thick it is. Rambos don't exist

But nevertheless, what makes me really, really admire IG is their sheer zeal and dedication to stand true against the forces that are MUCH, MUCH stronger and way more powerful than themselves. Only because they stand together like real tough SoBches they win many fights against all odds
Truly, they are flesh and blood of the Imperium

But still, they are only human and not supermutants with super-resilient psyche, covered with a thick layer of ceramite, or tall, cunning and extremely agile, teleporting for-seers; not to mention twisted warp creatures of Chaos that mostly make regular humans lose their minds at once.

I'm exaggerating a bit, so that it's easier to imagine and understand what I mean

Tabletop, card-game, Space Hulk and such are always way different.
It's the same if we would try to judge Game Of Thrones by it's tabletop or something

To me the only group that can be done properly is the Sisters of Battle. Imperial Guard are too weak individually to make much sense lorewise and Grey Knights are walking rape machines that have no possible balance without diluting the purpose of the Chapter.

I think it's a bit early to have this discussion.

The game won't lauch for a long time yet and we will have to wait and see how things work out with the four factions that are currently planned.

If and when another faction is added to these four, I think it far more likely that the choice will go to the Tau or Necrons, just because these do not come with the problem presented here. Because let's face it, Imperial Guard or any other Imperial faction that is not allied with the Space Marines would not make much sense.

And as others have already pointed out, the Space Marines will likely be a very powerful faction in terms of number of players to begin with.

Catachans don't exist? *joke*

Well actually the TT is also mainly about Infantry Battles. The Chimera is an Support Transport Tank, getting his Soldiers to the Frontline and giving some Cover. Than maybe 1-3 Leman Russ Tanks and you have the Common IG Army. over 60 Soldiers to 5-7 Tanks is not Vehicle based Combat.

The Imperium fights itself from time to time. If the Inquisition says the Marines have to be killed.. they will throw anything the Imperial Word has against them no matter what conflict there is at the Time. Can also work vice versa. And that the Inquisition only needs the 10% possibility of you being corrupted to kill you, your Family and your Dog is well known.

People seem to be using the phrase "Inquisition" when they mean Inquisitors (Ordo Malleus , Daemon Hunters) which include the Grey Knights.

The Inquisition also includes Ordo Hereticus (Heretic Hunters) which are the sisters of battle and Ordo Xenos (Alien Hunters) which are the Deathwatch.

So Sisters of Battle ARE Inquisition forces, they are the heretic hunters for the Inquisition, they battle the forrces of Chaos but primarily battle Imperial Guard forces and Space Marine chapters that have been found guilty of heresy by the Inquisition.

Still the TT is what started the Lore and is therefore maincanon. I still think we shouldn't go too deep into the TT Rules, just stand roughly by it. Two Armors with +4 doesn't have to be exact the same, just comparable and that goes for every Stat and Equipment.