All want from Forza 8 is not try and copy GTS. For me, I love to collect cars now, and reducing the car list just because some people want a GTS clone will be a stupid idea, if not awful. Not everything should revolve around esports or anything similar please.
This "Built not Bought" sounds interesting but I don't quite understand it, so T10 needs to explain more on that one. Locking cars behind "progression" isn't what I call "progession" anymore imo (if you don't know what I mean, take a look at FH4 for examples).
All cars should be unlocked especially for Freeplay Mode (the best mode in FM7), but in career mode you should work towards earning all of them, or build upon the ones you already have for tough events, etc.
What T10 should do, is make a vastly different Forza game but still keep the spirit of Forza in it (feature and content wise). Try mimicking the old Forzas. Because as far as I can tell, people just want a FM4 remake. This is where features and content comes in; gather everything that made Forza famous and built upon it - like Clubs and the Race Regulation, while adding new stuff above all of that.
A GTS 2.0 is not going to cut it. You will just be driving away all of your fanbase for hardcores who will just complain if there isn't any thing they want. Many already has a hard on Forza after FM7, so going that route wouldn't be a smart move. People seriously need to drop that mindset, really.