1. GT6 needs a proper, full career system - GT5's was far too short and there was very little else to do once it was finished. At least GT2 had manufacturer specific events and an Event Generator to keep you busy. There is a chance it could be patched in, but I believe the changes that need to be made (e.g. the removal of the XP system) would be too major to streamline into the existing system.
2. A significant improvement in sound quality. Most current sounds are too quiet, too synthesized, or just entirely wrong. Engines must sound louder, more accurate, and the gear changes less abrupt. The sound must be less clean and artificial. This couldn't be patched in, as the problems with sound design are inherent to the game engine and the recording techniques - a very significant amount of data would need to be replaced added - too much for a simple patch.
3. Better damage. The damage was very poor in GT5. Most cars seem to bend, warp, or smudge in collisions, but parts aren't lost, glass doesn't shatter, and roll-over effects are very poor. The detail should be improved, detachable parts should be added and a more convincing body deformation system should be added. Like with the sound, you'd never be able to patch this much data in.
4. AI. AI is very, very poor. It's not as bad as it used to be, but has been consistently a big problem with the GT series. Considering it's a racing series, the opponents are a big part of the game, yet the developers seem to consistently neglect this area. Opponents are too timid, too slow, and almost completely unaware of the fact they are in a race - it's more like they're competing in a time trial simultaneously. They need to be more aggressive, more determined (e.g. overtaking and blocking) and more aware of each other. It seems like the challenge comes from how far back you start and whether you can reach the front in time, rather than fighting opponents for position. Again, the flaws run too deep to be fixed with patching the existing design - the AI behaviour needs to be completely redesigned.