Here`s my take on customisation.
First of all, bodykits: The tuned cars in Prologue are good enough for me although I would like a huge pallette of non-standard colours plus the option of full body carbon fibre (with realistic weight reduction and extra cost, of course) Not too bothered about NFS style decals, but race car style designs would be good.
Exterior mechanical components: Different tyre brands/models that look and perform differently. Different exhaust brands/models that look and perform and SOUND different. And, ofcourse, wheels but GT4 catered for those good enough I suppose. Maybe some new brands/models added.
Now then, chassis mods: I think this is a contentious issue because if you mess around with this too much, you run the risk of irradicating any characteristics of the original car with regards to handling. I think just having the original chassis geometry and leaving it as it is in GT4 and giving us a -+ range to work with will preserve some of the original car's characteristics. Else why have a range of cars to work with when you could conceivably make an Elise handle like a Corvette?
Engine mods: As it is with the addition of a few things. Engine swaps (realistic limitations would obviously apply). Branded modifications with discernable differences. ECU software remapping. Just as long as ALL of the mods make the engine sound different. That is one of my biggest gripes with the series so far. Only exhaust/NA tune/turbine kit make the car sound different when things like port polishing, balancing and cam changes make an often dramatic change to the way an engine sounds.
I know this is mostly pie-in-the-stuff, but I live in hope.
Gary.