so pretty much anything, if you make it someone will buy it.
just look at all the things that are available for a honda.
after you get that list then make it for the gvr4.
suspension pieces (strut bars, supprt frames, bolt in roll bars)
Projector lights (complete light, diy retro fits are PITA)
engine dress up kits
cf body panels(fenders, doors, the works)