My first idea would be to give us more options for transmission upgrades, like give more transmission types and also allow us to choose how many gears that gearbox will have (4-speed manual, 10-speed DCT, etc.)
Second would be to let us tune our engines in the tuning menu (fuel mixture, compression ratios, etc.)
My final suggestion has to do with the descriptions of individual parts (Descriptions for upgrades). We used to have this in Forza Motorsport 1-4 but I’m not sure if we still have this in forza 7. If we do, let them stay. If not, bring them into Forza. What I mean by this is when you press Toggle, it will show a description of the specific part in detail (Sport air intake says ‘Install a cold air intake’, Race weight reduction says ‘Remove all interior carpeting’, Race differential says ‘Install a 2-way locking differential’). It shouldn’t be too hard to add these descriptions, they could literally just copy and paste these descriptions from the older games.
Anyways thats my list.