Hey dude, wow I actually haven't heard of this 40K mmo. But now you got me excited! lol
Yeah, I'm a big fan of the 40K universe and as for ideas, hmm....
Maybe it should be a mixture of the action bf2142 brings us, with more in-depth upgrades and feats using a leveling system. That way you could start off as a piece of cannon fodder and work your way up to become a customised 'hero' or avatar?
If a big open plan battlefield was presented with more than two armies in the fight, for example, bf2142 conquest with 6 sides competing at the same time as opposed to two, it could be interesting.
Back to BF2142 (I like the game ;>P), imagine titan mode, only you're invading a tyranid ship for a bit of terrifying CQB down a console corridor? That's a small wish list that prob won't be recognised - but hey, any 40K MMO sounds freakin awesome to me!