It depends. GT fans tend to spread their driving experiences out in areas that other racing game fans don't bother with. So I'm for darn near everything, as far as PD can go. As others have stated in this and other threads, Race Mod should follow certain racing classes and disciplines, so that if you RM a Passat for a possible BTCC conversion, it would produce a car fitting the specs for that league, and for WRC, a car suited to Rally specs. There can be fantasy GT Classes as well.
Livery editor is a must, and as with Forza, I'm for being able to paint liveries over race cars. Non-destructively, of course, and having a library of liveries for as many cars as we want.
I'm also for Tuner Modification, with their own class levels, as we had in GT2 and to an extent in Prologue.
Whatever we end up with, a Livery Editor is essential. Squabbling over paint chip colors in GT5 is so lame...