I've been thinking about this question lately, and while Forza 4 is still quite a ways off, I'm wondering... what do you guys want from Forza 4, whether it be features, or specific cars and tracks?
 
For features, I'd like to see weather and a day and night cycle. I really liked the weather system in Project Gotham Racing 4, and I think it'd be neat to see an accelerated 24 Hours of Le Mans event. I'd also like to see an increase in the number of cars on track.
 
As for specific cars, I'd like to see the Venturi Atlantique 300 and 400GT. I really enjoyed enjoyed driving those cars in Gran Turismo 2/Test Drive 6, and I haven't seen them in many racing games since. In fact, those are really the only two games I recall ever having them. The TVR Cerbera Speed Eight/Six would be cool as well. That's another car that was in both GT2/TD6 and hasn't appeared in many other racing games since then. Tracks wise, I'd like to see Circuit de Spa-Francorchamps. I really enjoyed that track in the Need for Speed: Shift demo and I'd like to see it in Forza.
 
That's all I can think of off the top of my head. What do you guys want from Forza 4?

I think there are 2 main areas to improve on - 
 
The first would be adding to the physics. The engine itseld is great, outstanding even, but there are some notable omissions like active aero (which is why the FXX in forza 3 has the top speed nerfed to 190, and why the veyron handles like crap) and higher downforce spoilers (maybe even wide bodykits, after all a DTM car is still an audi A4 underneath) for production cars. Seriously, take a look at the giant GT2 porsche wing compared with a road one, they simply can never match, but despite that turn10 combined the production and race classes.
 
As for gameplay, i think the single player needs to be a more immersive experience. When you race bumper to bumper right up to each other's maximum potential online, the pressure combined with the precision and thinking needed makes it amazingly intense. Overstep the mark even slightly and it's race over. Understep the mark slightly and he'll get through. You'll actually end up holding your breath and forgetting to blink. Career mode just feels like riding a mobility scooter in comparison. 
 
Oh, and it seems to me that they could add a rally mode quite easily. It's what, add some surface-suited tyres, input how a car reacts to gravel or mud or whatever... I don't know why you would stop at GT and LMP in the race categories.