So the thing with Mushroomy Kingdom is that it honestly feels like a relic of old "realistic video game" art from around that same timeframe of the internet. It works because it's an interpretation of vague NES sprites, so whatever other stages get this sort of reinvention should probably be similar in that regard. And the thing is, Smash Bros already has a few more examples of this... in Melee. Notably: Brinstar, Icicle Mountain, Fourside and Temple are all pretty derivative to the source material. So we can use those as reference for this experiment.
I think a good one would be Death Mountain from Zelda 1, but reimagined as almost DOOM 2016-esque in its presentation; vaguely sci-fi/alien with big, ominous structures in various states of disrepair. I was going to suggest something from Zelda 2 since you probably take more liberties, but again, Temple is kinda that already. Plus we could finally have a home for that Death Mountain remix from Ultimate.