GNX cluster as part of the regular GN package. you just know that, given that every other G body had a full gauge cluster of some sort available, that someone at some point was working on a Buick gauge cluster.
4 "front" GTA wheels as an option. they fit just fine and are already in the GM parts catalog. use the same 245/50/16 tires as on a same year IROC or TA.
bigger brakes, with optional rear discs.
GNX fender vents.
better flowing exhaust- high flow cat and maybe get rid of the muffler and just stick some resonators on the end of the pipes. but that wouldn't pass federal noise laws or Buick quality standards.
harder body bushings and install all the "missing" bushings.
all the frame and body bracing.