For the multiplayer, I was thinking simultaneous play. There are three options for losing at a level.
1. If any player gets in the way of an enemy, everyone loses.
2. All players must get in the way of an enemy to lose.
3. The second option but also points get decreased in a fractional manner.
Another idea I thought of was more of a last man standing scenario. Like maybe a giant level with enemies randomly spawning in more and more (random despawning could also be included).