Do you think that it would be possible for members from the Space Marine faction to fall to Chaos? I know, this git is trying to be clever and make your work load larger, sorry.

Only if it reset your stats

It's called logging out of your space marine character and making a chaos character

I doubt there'll be a complicated system for people to slowly fall to chaos

It's hard to do because essentially this is a one way street. If you fall to chaos, in the eyes of the Imperium, there is no coming back.

It would have to be something you choose to do with big red warning signs so people don't do it by mistake.

and even then people will do it and complain afterwards. So... not going to happen. At All.

Should not happen at any point or time, otherwise it would be unfair for other races who cant jump allegiance like that.

What kind of effect would it have on the Eldar?

I'm sorry.
As a multiplayer game with 4 factions I see no way this idea could fit in.

In a singleplayer game it would quite interesting.

Bioware Warhammer RPG? lol

Warlock: Look Farseer, a crude ork gun. How do these primitive things even shoot i wonder.
Farseer: Careful there Warlock, those things are tainted.
Warlock: Now dont be daft *picks up the shoota*
Farseer: Warlock NO!
Warlock: DAKKA DAKKA DAKKA! GREEN IZ BEST!