There could be bosses that have a lot of health and special attacks.
They would appear at waves 10, 20, 30, 35, and every wave 40 and beyond.
They would scale with the wave to be more/less challenging.
Multiple players would scale the boss's health.
Waves past 50 start to have enhanced versions of the bosses.
Waves past 60 can have two bosses at the same time.
At wave 70 you win. You simply win. Score is added for shorter times and subtracted for hits taken.
Boss rounds last 1/3 as long and end with the boss spawning after all enemies are dead. Doesn't end until the boss dies.
[ ] A big orb that sits at the edge of the stage with snake arms that shoots triangles. You can destroy and knockback both arms. Enhanced version has extra arms.
[x] A superlong "dragon" that shoots projectiles towards players from its mouth and likes to fly from the outside of the stage towards the inside. Enhanced version shoots laser beams.
[ ] A supermassively scaled blob that breaks apart many times. Probably a good early boss.
There could be bosses that have a lot of health and special attacks.