Improvements:
SuperBikes: PD needs to concider adding every modern superbike avivable,from 2000 until present, it should include all Suzuki 1000s, Kawasaki's, etc, some of those bikes are legendary and they are missing out. PD should also concider adding classic superbikes, like the old Ducatis.
NakedBikes: They also need to buff up on the count of naked bikes aviable.
On Off-road racing: I really don't think that PD should head in this direction, due to the fact tha GT4's offroad sucks heavily, the physics for a motorcycle on road and off-road vary greatly, and it'd probably hold the game back.
Track Racing: PD should honestly concider upgrading their A.I., although it's good, it's barely average, if you race the A.I. with the same bike they have, yours will always be faster, it's as if the A.I. is afraid to get 100% of the bike's power. It should also add more bikes to the race grid, and start from a stand-still.
Drag Racing: Seems like a nice addition, but it's just that, there's not much you could do, unless you would start to upgrade aerodynamics and add turbos, etc. Then if you can upgrade those parts, you should be questing for a stream-lined bike, long, low, and aerodynamic.
On Upgrades: PD should concider not only their current upgrades, but others such as superchargers, turbos and nitro, also concider upgrading better and adjustable steering dampers, carbon fiber fairings, stronger brakes, bigger variety of exhaust systems, engine components, etc.
On Moto-GP: I doubt they would do this, seeing as how Moto-GP has it's own game line, but it'd be nice if they had, like GT3 used to have the formula cars, TT2 would have at least some GP, or GP based bikes, even if they're not sponsored by the same people.
On Gear: Include other gear, like Johnny Rocket, bigger helmet variety, ICON gear, etc.