Pixel Art Stage: Lava Factory (Annalynn)
Holy cow, FazDude submitting Annalynn content? Who could have seen this coming?
In all seriousness, though, I think a stage based off this area in particular could be fun. I'd imagine it'd have a similar layout to what's pictured. This stage's main gimmick is provided by the Venom Brood, consisting of the snakes Randy, Handy, Candy, and
Satan Mike. They would enter the stage through the doors seen in the image and deal damage upon contact. However, Blinding Rubies will appear on the top two platforms on occasion; Once picked up, a fighter gains the ability to kick the Venom Brood off-screen via touch, as well as increased attack power!
In short, it's pretty much Pac-Maze. And who doesn't love Pac-Maze?
Underground Stage: The Underground (UNDERTALE)
We already have Frisk as a fighter and Sans as an assist, but I don't think UNDERTALE representation is complete until we get a means to display its world and music. The Underground would do just that; It would begin in the Ruins, then transition to Snowdin, Waterfall, Hotland, and finally New Home before looping back to the Ruins. Assume some SAVE shenanigans are at play here.
Puzzle Game Stage: Cobalt Plateaeus (Angry Birds 2)
This stage is All-Star Strife's answer to Smash's Super Mario Maker stage. While the general layout remains the same, pig fortresses are randomly generated at the start of each match. While usually acting as walls, they can be toppled over by strong attacks or being launched into them, potentially creating a lot of chaos with the resulting debris (even if the actual falling down isn't as fluid as the source).
(this might just be my most self-indulgent set of submissions yet)