Commodore 64 would be cool, as would TurboGrafx-16, provided they used many of the PCE-only titles as well.
There are quite a few mini bluetooth keyboards available now for under $20 retail. They use a single membrane and one big sheet of keys like an old cell phone. Cost wise it could make sense. AtGames does not sell their stuff overseas though, and that eliminates a big market for anything C64, in Europe.