With online gaming definetly coming into the GT series, i think GT5 needs to be much more driver oriented, rather than car focused. I would much rather see how it really is like to walk inside my garage or the pit lane than having 700+ cars. The PS3 will definetly allow this. I would like to have the ability to personalize my driver - sometimes i don't wanna be wearing an ancient helmet and funky glasses just because i'm driving a chaparral 2J. I want to choose my helmet, maybe from various pre-loaded designs or, like someone suggested, transfer them from USB flash memory (although I think PD will reject this). Maybe even put some stickers in it.
When I say they should improve the driver aspect, I really mean it - you should literally see your character everywhere, and control him (or her). When you sign up for a race, you should be able to walk into the pit and enter your car, watch the pit crew do their job and start the race when everything is ready. And don't forget to include photomode with that. Most people will say that this will make the game more complicated and so less accessible, but I think this is the kind of atmosphere the GT series is lacking. You may also say that this kind of process will be tedious for 2/3 lap races. Well I think that kind of races should be abolished. The game should be harder and longer. It is not, in my opinion, just a driving game that kids can play. In real life 2 lap races don't really happen on a daily basis.
Online races should be more of a social event, where you first meet the other drivers, not the other cars, and then race.
I think a very nice addition would be something like what was on the original Perfect Dark game on the N64 - you could take a picture of your face and put yourself in the game. This will be perfectly possible with some sort of EyeToy device or by transferring from USB memory.
Another cool addon would be headset support. Not only for online races, but for talking to the pit crew during actual races in GT mode. With some kind of voice recognition, you should be able to ask what lap you are currently in, how far behind you are, etc. Besides being cool, this could simplify the HUD without making you unaware of what's going on. It would also be an excellent way to control B-spec if the driver could talk back. And before you say this is impossible, I assure you it isn't. If I remember correctly, SOCOM has this.
And if the "driver element" thing is going to be for real, we're gonna need an in-cockpit view. With the interior completely modeled, even if it means sacrificing some cars (Skylines...).
I would also like to see garage cars in LAN / Online races, and the ability to use 3 screens for GT mode, and with only one PlayStation (I guess that depends on Sony and how "revolutionary" they want the PS3 to be 👍 ).
A clutch pedal is something I have also been waiting for. The next DFP should have this, and the pedals should be way more resistant. The shifter could also use some improvement..
I also share some of the opinions already posted, as I would like to see weather and day/night changes, faster tuning, much better AI, and I agree that damage should not be included.
And just because I didn't give much attention to the AI in this post, doesn't mean I don't hate it as it currently is.
Oh.. and I would also like to see karts in the game... Please...
EDIT: I forgot to mention how IMPORTANT it would be to have at least 12 cars on track... That would make the whole atmosphere thing much better and realistic, and the game would be more interesting, especially during long races. 👍
EDIT 2: Two more things: exotic cars (we all know what we're talking about - historic ones too) and something someone has yet to mention: 3D RIMS during races!