In addition to what was already said,
1)Different exaust notes from different manufactuer's exuasts
2)Adjustable boost pressure
3)Clutch
4)Brake fade
5)Engine swaps and rebuilds(finally be able to have a RX7 with 26b and 240sx with RB26)
Thats my wishlist.
BTW, GT4 does actually have Porches, autoRuf. But more selection of porces would be better.The only reason they have flat textures on the wheels while racing is to keep poly counts down.The wheels you see while not racing are costly in polygons,thus limiting other details.And unless they make it for computer then the posibitlity of DL'n other cars is imposible, unless you copy them to your memory cards.