here is a question how would you improve stage builder?
I think a better stage builder could be made by just expanding functionality of existing options and a couple of editing features.
1) Allow switches and rails to be detached from the objects they modify.
2) Allow multiple objects to follow the same rails and switches
3) Make an option for thin platforms to act identically to those in official stages (so projectiles don't collide with the side walls)
4) Give an option for motion and respawns to act on timers so they can have phases
5) Add copy/pasting for objects you already made
6) Somehow fuse aligned objects so the edges don't act in unexpected ways (as it stands you can up-B into a wall made of two pieces and it will act like a ledge, allowing you to up-B again. Floors made of multiple pieces will interrupt grounded projectiles and I think stop dashes)
7) Maybe give an option to allow platforms to move through each other (this is the one I think is least likely)
They could add some extra special pieces like cannons for fun, but I think some tweaks to the existing formula would make a bigger difference and allow people's creativity to shine. As it stands now, a lot of cool stage ideas just can't work, no matter how clever you get.