Rubber/Vinyl floor mats. The OG translucent colored ones that you used to find in original cars years ago. Have not seen those in many years.

Pushbutton radio buttons. Chrome is always pitted or flaked.

Brake splash shields.

70-71 Dodge Challenger bumper guards. Someone on here was looking for those and turns out they are hard as hell to locate.

Radiator support to k-member brackets. Often full of gouge marks from hitting concrete curbs or wheel stops.

Brake rotor outer wheel bearing dust caps.

Correct chrome rallye wheel lug nuts.

Rallye wheel trim rings. Don't even think you can find 14" ones anymore, and clean 15x7 ones are worth stupid money now.

OEM style lower control arm sway bar brackets that can be welded to non-sway bar control arms. 67-72 a-body lower control arms with sway bar tabs are not cheap anymore.

Heater box housings would be cool, but doubt that would be cost effective. Many are dried out or cracked so it's getting hard to find nice ones if you building a high end car.

Mod tops and gator tops.

How about 904 and 727 oil pans? Maybe make them out of thicker metal too?