I think they should add a couple of wide arches to every car
For example JGTC style ( front wing with side vents with bulging rear)
Bolt on style
Or what they have done with the BMW 850i with an OEM plus style wide arch kit (wider but flows with original design) probably best choice
All dsigned by Forza so can be used with other aero parts such as front and rear bumpers without breaching licensing. Even if no other aero is available have these or at least one of them available for each car