I'm not sure it's worth the effort to work on an fpga gba when the woozle gba consolizer is very close to being released and used gba's are cheep and plentiful.
PS-One seems unnecessary because they are cheep and plentiful, we already have the PSIO and great rgb, plus an hdmi upgrade might be coming from chriz2600 and citrus3000psi. Not to mention PS2/PS3 options.
N64s are still affordable, have the everdrive and ultrahdmi
Dreamcast has USB-GDROM and good rgb/hdmi options.
Sega Saturn has the Satiator coming soon, good rgb, and could also be ripe for an hdmi mod like the dreamcast.
So that leaves only a few options...
I'd rather they spend the next year figuring out a full fpga 32x or SegaCD solution to go along with the mega-sg because all the current solutions suck.
Otherwise, I think either the neo-geo or pc-engine would be great. Both are kind of hard to come by and so an affordable way to enter that world would be great. A lot of people would end up buying one because they have all the other consoles and so why not pick one up. I know I would.
Atari Jaguar Maybe???
Here's a more exciting option. Release a handheld supernt, megasg, or ntmini. I thought about buying an extra supernt or megasg just to do a handheld mod. The motherboards are relatively small, power efficiency is decent, and finding a screen would be very easy. If only I had time to work on something like this I would, but Analogue could just save me the trouble and take my money.