Sonic & Mario: All Star Jam is a game from an alternate universe where Nintendo bought SEGA in the N64 era. It's a game compilation featuring:

Sonic The Hedgehog
Sonic The Hedgehog 2
Sonic The Hegehog 3 (& Knuckles)
Super Mario Bros
Super Mario Bros 2
Super Mario Bros 3

All of the games are remade from the ground up, with the Mario ones being based off their SNES All-Stars remakes and the Sonic ones basically the same as they were in Sonic Jam

The music in all the games uses the N64 soundfont.

Every Sonic game has one Chaos Emerald missing from them, all of them being hidden inside the Mario games in new secret areas.

Once all the Chaos Emeralds are collected, a hidden Mushroom Kingdom Zone will be accesable in Sonic 1, and Mario will be unlocked as a playable character in all 3 Sonic games.

(This is just a random idea I had and decided to quickly throw together. Might make some more advanced mockup stuff like a better logo and maybe even menus)

https://www.deviantart.com/versquigium/art/Sonic-And-Mario-All-Star-Jam-Box-Art-Mockup-849817509