When I mentioned using older designs I simply meant for ease and speed of build. There are a few that would still be useful, like an external flicker fixer/scan doubler since Indivision is in short supply these days. Plus you have a bigger market than just the 1200.
Anyway, additional ideas;
Pico Power supply adapter (Internal or external)
Internal speakers and amp kit (made one of these myself with two mono amps from the pound shop!)
loving the idea of the new cases and/or keys for those people doing paint jobs
how about a little graphic equaliser?