There is enough content to be a 0.0.1 version. I speak in mention of the number of selectable characters, and I have several ideas and things to improve:
1. It could give them the opportunity to give them skins for each character as it happens in Super Smash Bros series: As example Classic Mario to Modern Mario, Classic Sonic to Modern Sonic, Gameboy Kirby to Pink Kirby, Ms. Pac-Man to Pac-Mom, etc.
2. That each character had their own invincibility theme from their respective franchises: Sonic with his own theme, or Kirby.
3. As this game, it references either Mario Crossover or Sonic Boll, you could be given the opportunity to add characters like Megaman, Tails or Knuckles.
4. Tetromino is broken, its block-by-block movement is strange and not at all fluid for a sidescroller (which in the original game made more sense because it was a fixed camera).