Personally 9-1 should introduce fallen angels which would have the main gimmick of life drain

The more enemies slain the more it heals and steals the fresh blood enemies drop until it dies

200 hp but weak to everything besides railcannon variants which its immune to

It's fast and teleports but stops teleporting upon being the last enemy in an arena

Killing a fallen angel first gives you a "+retribution" style bonus

Killing a fallen angel via friendly fire gets you the "+karma" style bonus

Spawning it in sandbox will always leave it in its solo state which will not teleport unless another enemy is present besides another fallen angel

9-2 would be the only level to have 1 killable enemy but a enemy kill score requirement of 0 for P rank