Infamous Aero make body kits for several cars already in Horizon 5. They also make custom spoilers and roof wings.
New Edge Mustang (SVT Cobra R '00 and SVT '03)
Toyota Supra A90
Nissan Skyline R32 Coupe
Toyota GR86
Subaru BRZ
Nissan Skyline R32 Sedan (not in Forza yet)