Here are some things i would like to see for car customization in forza horizon 5:
Steelies
Aftermarket wheels for trucks (ie Fuel Offroad Wheels)
More body kits (ie Yashio Factory, Re Amemiya)
Exhaust tips
Aftermarket steering wheels (ie Momo, Nardi)
Ability to adjust the amount of flames coming out of the exhaust
Air Suspension
More camber
Window stickers
And lastly
Lift kits (for trucks and SUVs)