Add some more tracks, some more PREMIUM cars and dump the GT4 cars entirely, change the GUI, go nuts with single player events, resolve the sound issue, dial back up whatever needed to be dialed down so the PS3 wouldn't melt down, DONE!
For many of us, there really aren't many games to play on PS4 until a GT game is released to justify purchasing it. Sure my kids want to play some games. Yes, there will be gsmes to kill the time while waiting for GT. I've been "testing" my cousin's PS4 before I ship it too him. NFS, I'm not impressed with at all. The free to play games do nothing for me. I suck at shooters, so I have no enjoyment from Battlefield 4. I don't feel that "gotta have it" anxiety anymore.
With the PS3, nothing really satisfied me until GT5p released. The wait was pretty agonizing. GRID, MotorStorm, NASCAR and other games didn't really scratch that itch quite right.
I think PD is in a better position to release a PS4 game than SMS is with PCARS, in a year from now.