I don't know about RM's for EVERY car. I would like to see 2 to 4 RM's for a select amount of cars. For example, the '10 Camaro can have its current RM, a GT3 RM, and a Stock Car RM. Old Mini can have a Touring Car RM and a Rally RM. As for course maker... we NEED the ability to tweak single corners and straights, not track sections that consist of over 8 corners.
A tad more backfire would make things more epic, along with being able to see OTHER cars' backfire ONLINE. Better replay options, a lot better. More photomode freedome, multicar shoots, and such. What would be really cool, in my opinion, is how they did the "indy pit stop" photo location with AI's driving the cars and you snap a pic. If you could choose your B-spec Bobs, some off your cars, and any track. Then you can scroll around the track while the Bobs battle it out.
Also DNF's from too much damage would make the game feel a whole lot more real. Cautions for the higher class races (touring cars, open wheel, stock cars). AI's that make actual mistakes like over shooting corners, losing grip out of corners because they jumped on the gas too quick, cutting each other off that results in another AI in the wall, I can go on and on.
Tire punctures, flat spotting, and tire ruptures would be epic also. You rough up an AI or another race online and cut a tire down for it. The obvious would be livery editor, I don't need to go on about that. Oh, I would love for there to be a driver options, like changing the height of your driver. I feel too short in the in-car view to be comfortable enough to use it all the time. Raising the height would make in car much mor comforting than it is. Also widening the field of view for in car also would be great. I know a multi-screen setup would help but can't afford that.
Lastly, online mini seasons. I loved how in DiRT2, you could select some tracks, restrict the class, fix the settings, and do a 5 race mini season to see who comes out on top. This would make public rooms WAY more apealing imo. Can't think of much else right now so I'll leave it at that for now.