I find it strange that most wishlists consist of more cars and more tracks, when surely the most important improvements for future games will involve changing the parts of gt5 that make the gameplay less rewarding than it could be. For example, I would love more realistic crashes, both in terms of visual damage and physical damage, i.e it should be possible to crash badly and have to retire from a race. Then winning a race would be much more of an achievement.
I also think the tracks shouldn't have the invisible walls at the edges. So if you ran off course you could crash through barriers/ropes, hit individual trees etc.
The events should have entry requirements, eg min and max hp or pp, weight etc, rather than you being able to just upgrade your car in order to win races that are difficult.
How about reduced grip when you stray off the racing line?
Penalties for cutting corners or taking another car out unnecessarily, yellow, red, black flags.
Less random mixes of cars in races (unless they introduce classes).
Qualifying, too!
OK, so that pretty much sums up my wishlist for future installments of gt. And lastly, I'd like to say that I love the game and it is very close to being incredible. As has been stated elsewhere, it's a ten out of ten sim in a five out of ten game! (though after the downloads and updates its more like a 7 or 8 out of ten game).