GIF Source: Vincent Buso
Look at it, you want it. And you can build it.
YouTuber Vincent Buso uploaded this short video demo of the Keymu. What you see is what you get. It's basically the clamshell design of the Game Boy Advance SP shrunk down to an ultra-portable keychain size. It has four buttons instead of two and it runs ROMs using an Intel Edison chip.
The developer was inspired by a project from the 2016 Hackaday superconference that used the form of the original Game Boy. The Keymu is quite a bit more polished and the extra buttons mean you could play Sega Genesis and some Super Nintendo games.
In the Hackaday spirit, the developer is also uploading detailed instructions for anyone who wants to take a crack at building their own. So far, they have only posted an introduction, an image gallery, and one blog post. But more is promised.
I could be wrong, but if some enterprising individual wanted to figure out how to manufacture these for a decent price, they'd have a hot item on their hands.