I was really psyched about the retron5, but now I'm psyched about this!!!
https://www.kickstarter.com/projects/ro escription
https://beatscribe.wordpress.com/2015/0 o-emulate/
Its just a Pi inside you so could do your own mods pretty easily.
I think the Game Kid looks cool too, but since it's just a Raspberry Pi that can run emulators, it's hardly the same as a Retron5, so not really an alternative.