I'd much rather see different tyres be added as well, the first racing game I played that had this nifty feature was GT6 and the strategy that comes with it is so much fun, it makes the races so much more exciting and a bit less predictable.
Honestly I don't think it will be in Forza this generation of consoles anymore. The very core of the game has been the same in FM5, FM6 and I don't think FM7 will be different. Actually FM7 might very well be the last FM game for this generation of consoles. Following the current release schedule fall 2019 would be the time for FM8, there are already rumors the PS5 would be for 2019 (
https://www.gtplanet.net/analyst-predicts-playstation-5-will-launch-early-2019/). So if Turn 10 has any plans to make any big changes in the Forza engine / core game then I'm sure it will be most likely be for the next-gen games, so that whatever changes they implement are more future proof.
Judging by the car list FM7 already feeven, a bit like a "greatest hits" like someone mentioned on this forum, thinking it was to impress the PC community. It feels like a greatest hits to me as well but I'm more thinking along the lines of the end of a console generation.