There is a few things that i think would be nice, but i want to know if they exist already/being worked on

Favourite Characters/Character folders, to be able to custom order your character roster
Actual custom stages/stage editor, meaning platforms to interact with
Functioning increased battle sizes, for 1v2 or 2v2 or 4 player FFA
Extra match settings, to change things per person, such as super gain rate or starting level, max hp, damage multiplier, ect, could do a fun little raid boss mode with that by having their max health start high and setting it to what it was on at the end of the previous match
On that note, detailed UI? like a switch that tells you HP numbers,, shows hitboxes, stuff like that, letting you learn about things better?

And these are likely external tools, but worth asking
An actual character editor for making custom ones
Split window, so if you only have once device but two screens and are playing local, you can see predictions without knowing what the other would do

Im sure some exist, but its just a bunch of things i think would be useful in a few different ways for people, i know the modding scene has been going crazy and i would love to be able to make a large folder of custom characters with their own music tracks for adaptive music based on who is winning as well, i could go make a game for this, but at that point i feel like im stepping on toes as this is such a good game, but i dont understand this games code well enough to add to it as is