I think a psychedelic or weird stage like Alone or Stimpy's Bellybutton would be good.
I know the Breadwinners don't have the best reputation but the Rocket Van would be a perfect stage - fight on the rocket, go beneath it to fight on the cramped hood a la Corneria's laser, there's a little incline on the rocket with a cave of life, and dangerous fire you've gotta avoid coming out of the rocket. Just make the van a little flatter and the rocket a little longer so the rocket can be a cave of live with the hood, and add some bread slice platforms that fly in and out a la Big Blue/Umbra and there's a solid arena.
There are so many good choices for a Rugrats/Reptar stage, but the playpen in a similar style to Living Room sounds the best to me.
The Mr. Meaty deep frier would make a fun stage - the grease is basically Brinstar lava, and you have to fight on meat (and other animal body parts) that emerge and submerge while the puppets try to pick you up and serve you - hell, the same concept could work for the Krusty Krab or Chum Bucket, though for the prior you'd have to make the food a lot more apetising.