With FPGA Arcade and TiNA we have two very good proofen concepts already, I do not see much sense doing anything different. TiNA is not a special retro development so it could be really affordable and I am already looking forward to it. Something nice (additionally) would be a cheap mobile device in future and of course we have emulated environments already including Android. So I think already a pretty nice list :-). At least in the near future FPGA is the way to go...