Nostril-esque hood vent/scoop that can be adjusted to have bigger breathe and longer runs.
Hood/bonnet wind louvers (whatever they're called) instead of just the one standard style, maybe a split version or etc.
Front grill like a Jeep. (maybe with choice of 5-7-10 holes and adjustable sizes)
Front spoiler/air splitter that is angled down at 45 degrees (maybe even variable angles)
Rear windows to showoff engine (also rectangle/square/adjustable).
Rear window louvers/vent/shades when window is so big it helps to have shade.
Wheel arch addons (like riveted or bolted patterns, & plastic {diff. materials, etc.})
Sunroofs, and maybe even T-top mods?
Roof lights (like on semi-trucks) (also found on some full sizes)
Side windows (rectangle) that can be stretched at either ends axis.
Side vents with more variety.
Side view mirrors (if even possible) (many versions of that)
Side skirts like ground effects 2000-ish camaro had, "RS" package with pieces all around vehicle, not just the sides.
Door handles and keyholes that match.
Trunk handle and/or keyholes.

Just throwing suggestions out there.