It’s possible they might be waiting for the Z06, because the C7 Stingray they added to Forza a few years ago has already been left out of several titles and replaced by the Z06 and ZR1. The modern Stingray has become too basic for Forza I guess?
If they add the Corvette C8 to FH4, then they should also consider adding the 911 Turbo S (992), F8 Tributo, SF90 Stradale, Aventador SVJ, M8 Competition, etc. At some point they have to draw a line and save something fresh for the next game. I’d be surprised if FH4 gets more truly “new” cars, but you never know.