Heater box cases would be great, but pretty complicated to do I imagine. Nice heater box cores are not that easy to get if you have a badly broken one. The ones in the junkyards here are typically dry and fragile. New ones made in a better material would be super.

Pushbutton radio buttons?

Bumper guards for some cars are difficult to find. Most bumpers are reproduced at this point I think?

1964 Plymouth grill guard / bumper over rider?

Brushed 14" trim rings available? Seems most everyone wants 15" or bigger wheels so this may be a money looser?

Headrests?

1970 ESC gas caps. Think NOS is all that exists for some types of gas caps?

Sunroof parts like weather stripping ect.?

Luggage racks?

Brake drums that are no longer made, or made poorly.

Small bolt pattern a-body disc brake dust shields maybe?

Recall Wheels? I think someone made them and then quit?

Any unique master cylinders need reproduced?

SB 90 degree oil filter adapters are now a lot more then they used to be for some reason. Are they now hard to get?

Any steering wheels?

Some alternator pullies made back them don't look like the ones that come on alternators today.

Wiper arms or wiper blades?

Sway bar tabs for lower control arms.

Pinion yoke dust shields.