Here's a couple of ideas for you that I would be keen on:
1. A carbon fibre look bonnet. Fiberglass with
just a top layer of carbon fibre matting to give it the look. You'd be best to copy a GX bonnet as they are the most sought after style. Or possibly copy a standard bonnet but add in a subtle scoop or cooling vents for the turbo fans.
2. Some fiberglass door trims that include a nice speaker hump for say a six inch speaker so you don't need to chop into the door metal to mount the speaker.
3. A replacement for the dash instrument panel that is made to fit a Silvia instrument cluster (commonly used for CA18 and SR20 conversions).
4. Some sill panel covers to replace the little aluminium things that are always dinged up.
(not sure if fiberglass would handle being stepped on though).
5. There's always demand for flat tops for the utes as well if you could produce one at a reasonable price.
Wow you're going to be busy..good luck and keep us all posted with what you come up with.