Given the relatively poor screen quality of the Gameboy (my memory is foggy, but seems to remember ghosting and generally bad refresh), I wonder about creating an E-Ink Gameboy Classic...
While it's a GBA clone instead of the original Game Boy, there's the K1 GBA that can play original carts and has a cartridge that will accept a microSD for roms. I was really interested in them a couple of years ago but then I found emulators for my PSP.