Personally I think the graphics were pretty good already, what would make a nice addition (and what some others lack) is "real" action at the track. People walking in the pit lane, photographers following cars, dedicated autograph hunters, the lunatic who has to run across the pit lane as you're coming in, haha. They did the modeling for the pit garage sequence, why not take it one stage further, how about getting ready on the grid? I suppose it's that "immersion" thing again.
Raceroom has some of this and it makes a big difference for me, it made me smile the first time I went to the pit lane to see people active on the pit wall, and seeing the marshals moving at their posts.
Apart from that, the one issue I always have with CM games (and it gets mentioned a lot already) is the playability. It seems to vary year on year as to whether the car is nice to drive or totally impossible. They seem to put so much effort into everything else and yet this is where they suffer.
I can't remember which game of their's it was but I gave up on one in the end, it was ok(ish) on controller but impossible on a steering wheel (just too sensitive). In the end I managed to dial down the sensitivity enough so that I could go round corners but you were only turning the wheel a few degrees.
The opposite side of that is the Gran Turismo games, an example of how the handling "engine" should be done.
The server side for PC will not happen, CM are just not interested in it. Which is a shame for us and a mistake for CM. Saw the entries for the formula 1 style iracing championship last night. 60 drivers entered...I hope I'm wrong and the CM team add it though.