I think the Raspberry Pi or Beaglebone would make good candidates for baremetal emulation. The Pi has a formidable but barely documented vector processor, and the Beaglebone has the (also under-supported) PRU realtime subsystems which would be ideal for controllers and audio.
Both also have enough ecosystem to make it worth doing. Retropie is already popular, although it's a straight port of the normal userland emulators and doesn't attempt to do anything fancy with the hardware. I think there is a "market" (not the kind that pays money, but a decent userbase) which will happily dedicate a cheap SBC to booting straight into emulators.