Bring all of that back, in one division, like GT4, with tuners like HKS and Re-Amemiya. But for each manufacturer that have an in-house tuner, like Nissan and Mercedes-Benz, the tuner is a separate part of their dealership, kinda like GT2.
For instance you can't fit M Sport alloys or wings to a Honda Civic or a Ford Escort, there solely for BMW's. On the other hand you can fit BBS or OZ alloys on any car. That's how it should be in my opinion.
Sounds too complicated, for sure.