The Creator

I'd recommend changing his name, calling him The Creator makes it seem like its a final boss thing. Also the location is kind of random for him too, he should have his own dedicated area instead of the basement of the house.

So my suggestions would be these

1. Change his name to Prince Pollywog or something since he uses those as attacks. Move his statue entrance to another part of the map, maybe make a biome for the pollywogs like a swamp or something. You could still use the cave theme for him by having his statue in a cave in that biome or part of a fountain, I'd also tweak his arena to fit the pollywog kingdom theme a little more. Mostly speaking decorations wise, the floor being yellow green like it is currently looks good. Just needs stuff to make it look like a throne room and not just an open area

2. If you move his fight/statue you can replace the basement boss with something else. Like you said in the dev video its pretty rough as a first boss encounter anyway. The new boss there could be a giant basement centipede (  I forget what the proper  insect name is ) and give it some interesting attacks. It can be reached in the same manner as the Creator, by that I mean you go down a hole and drop into its lair

For Toilet Realm;

Give plunger guy a proper arena. Something that looks more industrial like rusted metal/boiler room vibe. The platform he stands on can be a pipe with a metal grate over it. This way you could also add a gate that comes down when you enter the area so he doesnt roll out of the room and mess up the cutscene.

I would also add something to reduce the chance of epilepsy during the Swirly D encounter for people with photosensitivity