Turtles in Space!
This is a classic SHMUP with up to 8 players. HP on left side, Score on the right side. When the score reaches the maximum, you reach the next stage. Enemies get stronger with increasing score. There are three types of powerups.
It was quite chaotic with 8 players, but it worked and I like how it scales up the enemies. Will have to try it single player to get a better idea of how it plays as the 8-player session escalated quickly!
Nice idea for a multiplayer game - I'd love to see more types of enemies and maybe some terrain as well to limit your options!