I like the idea of a story mode, progression, etc. You could probably come up with a lot of fun bosses. Some quick brainstorming:
Every X turns, triggers a bomb which damages nearby players/items/enemies with 3, and takes 1 damage itself. Immune to fireballs?
Every X turns, kills a nearby enemy, but gains the HP itself.
Every other turn, it expands, revealing spikes which doubles the current HP of the boss (basically a spike trap)
Every X turns heals nearby players/enemies/items with 1(?)
As long as it doesn’t “kill” the endless gameplay. Two modes – story mode, and classic (endless) mode, maybe?