I think it would be cool to add a alien monster thing. it could be in a cage and its limbs hit the robots or something like that.
Add lasers, wall turrets, falling/disappearing floor, mines, etc.
Fun ideas! Maybe a tentacle that grabs people and squeezes them apart? :
First thought reading this was a giant squid in Super Mario Sunshine. In the harbour area.
While in that mode you pull off the tentacles, perhaps the horizontal slash could take off the tentacles while rotating or something like that.
The squeezing idea sounds like it would be an insta-kill unless you have some "degradation" concept planned. Choreograph also sounds like it may be difficult.
yea like this.
maybe the spider-tron 6000. instead of throwing bombs at you it try's to hit you it could have two heads that you need to kill.