Famicase is an annual exhibition of Famicom (Japanese NES) cartridges for games that don't exist. It's been going on for many years now and accepts submissions from all over the world. If you click on a cartridge you can see it in larger resolution, along with a description of the game (about half in English and half in Japanese), and details about the creator.

Normally there's a physical exhibit in west Tokyo where you can actually see all the cartridges. That didn't happen this year for obvious reasons, but you can still enjoy all the cartridges online for this year and previous exhibitions.

There's also a game jam called A Game By Its Cover where people make games based on the cartridges. This is not run by the same people who manage the exhibition and is conducated mainly in English, you can find more details on their Twitter:

https://twitter.com/agbicjam