How about a challenge where you have to survive as long as you can?
Depending upon the starting level of the challenge you end up with spawns comparable to Weapon Shipment and the original Servants of the Overlord end sequence. Now since lag is a consideration, it'll have to be increasing CR mobs instead of increasing numbers.
I can picture the CR30 runs being full of Doomspheres, Ghaele and Drow Priestesses. Maybe some Mimics thrown in for their slowing attack.
The number of stars equates to the length of survival.