Rieger is a German tuning company and bodykit producer, founded in the year 1987. They specialize in modifying European cars, and their kits range from basic lip kits to complete widebodies. Forza used to include their bodykits on many cars, mainly Volkswagens, in Xbox 360 titles, but since the switch to Xbox One and beyond they haven’t returned on any but two cars - the BMW E34 M5 and the Volkswagen Golf R Mk6 (mislabeled as Abt).
Here are some cars that should feature their bodykits:
Audi A4/RS4 B5:


BMW E30 3 Series (not the M3):
BMW E39 5 Series (after a remodel):
Mercedes-Benz W201 (after a remodel):
Volkswagen Golf Mk6 R:
Volkswagen Golf Mk7:
and the R version: