So I tried it out and it's a nice start. My suggestions would be:

1. Needs way more documentation. It's not clear how to do anything. Doesn't explain how to upload games, even what platforms are supported (I guess it's just gameboy), doesn't explain what the controls are anywhere, has a feature to use your smartphone as a controller but doesn't explain anywhere how to do it.

2. For Super Mario Land the volume didn't work.

So I would start by further documenting what you have. And I would add a way to customize the controls and support the gamepad api. Nice start though. I'd use a service like this that let me upload any arbitrary gameboy, nes, and supernes games and not just specific ones.