A while ago I had an idea for a Smash game where you could choose to play your character as they have always been in Smash, or to swap them over to an optional, alternate moveset with an entirely different playstyle that could reference other games in their series. Some of my ideas were a Cappy-centric Mario, a Poltergust-centric Luigi, a Perry & Vibes-centric Peach, a Wario Land-centric Wario, a Dungeons-centric Steve, an actually game-accurate Ness, and an actually game-accurate Ganondorf.
I know this would never happen, but I still thought it was an interesting idea.
-Prince Of Persia
-Portal
-Angry Birds
-Sonic
-Mario
-Minecraft
-Overwatch
-Spider-Man PS4
-An undirect tie-in with the Avengers game
-Undirect Arkham tie-ins
-A good chunk of Midway Arcade classic IPs
-A Space Invaders LEGO piece in an Avengers set