GBA would be nice, but honestly, I think the first we'll see is gameboy, nes, sms and other 8bit systems before anything else...
Hopefully a Genesis emu gets done soon, as I would like that too...
The thing I would like the MOST is PC-Engine CD, and PSX for Rondo of Blood, and Symphony of the Night...but I doubt I'll see that for a long time, if at all...