I like most of the car lineup in FH3, but for FH4, I would definitely like to see a few additions...
First, keep the Aussie muscle cars no matter where the new game takes place.
Add more convertibles...I never did understand why Forza has so many hardtop versions of cars that look better as convertibles. Maybe have an option in the upgrade shop to add or remove the top. These are a few I'd like to see with a convertible/targa option...
Ferrari 365 GTB/4 Spyder (with wire wheels please)
*** Ferrari 308/328 (why Forza has never included this car is beyond me, it's one of the most famous Ferrari's ever made...Turn10 PLEASE add this one in the next game)
Ferrari 348 (another overlooked Ferrari)
Ferrari F355 (GTB, GTS, and Spyder options)
Ferrari 360
Ferrari F430
Honda S2000
1st gen Miata
BMW Z3
BMW Z4
BMW Z8
BMW 2012 Zagato Roadster concept
Mazda RX-7 (FC)
72 Chevy Blazer (FH3 has a Bronco, so why not a Chevy)
67 Chevy Corvette roadster
Austin-Healey Sebring MK5000
67 MG MGB roadster
Triumph TR6
TVR Trident
TVR 3000s
TVR SC3
TVR S1/S2
Other cars to add...
1985 Toyota Celica GT-S
1980 Toyota 4x4 truck
1966 Buick Skylark GS (too many Chevys in the game)
Ferrari 308 GT4 (why not?)
I would also love to see a few more wheel options, including some from American Racing (80's-90's designs), classic Ansen Sprints, and anything with a deep lip that looks good on older musclecars.
"I knew I should have brought demon chasing shoes!"