I would really like to see wide arches available for lots of cars, but only when offset/wheel width is added too. possibly even branded types like TRA_Kyoto, Driftworks, etc. It could be included as an aero part. On the point of aero, I would also like a broader selection of aero on a broader selection of cars but that's a different thread!
Factory extras for cars from the companies like TRD and Nismo would be great too, like a TRD Laguna kit for Altezzas or N2 kits for AE86.