Alright, hear me out.
Is it just me who wants more costumes for characters we have?
One idea is to have characters have 'retro' alts. I see a lot of mods for Smash 4 that give Cloud his FFVII model - so stuff like this is clearly popular. Perhaps you could use models from Smash 64 to start with, as well as some extras (some ideas that spring to mind for me are Mario and Luigi having 3d models of their Super Mario Bros sprites, which would be awesome.) Obviously this wouldn't work for every character - specifically characters that are more recent, like Joker, Shulk, Olimar or Bayonetta, and characters who's sprites wouldn't really work for their hitboxes - like maybe Hero (although the Luminary is new, Dragon Quest XI does have a mode where you can play like a traditional RPG.) Only some characters would get it, and I suppose it would be like Little Mac having his wireframe alts - but if you didn't want them, then you don't have to buy them. They'd obviously be cheap - there's a lot here, but I suppose you would buy them in a pack or something, perhaps called the 'Costume Pack?'
So, here's a few ideas I had.
All of the veterans from 64 would get their 64 costumes.
Mario, Luigi, Peach and Bowser would get SMB sprites.
Dr Mario and Daisy would also get sprites in this style.
Mario and Luigi also get sprites from Super Mario World, as does Yoshi.
Link could possibly have sprites from Zelda II? I'm thinking of proportions here. He also gets a sprite from Ocarina of Time.
Samus gets her original alt from the first Metroid.
Zero Suit Samus gets the original 'Justin Bailey' alt, in pixel form.
Dark Samus would also have a pixel alt in the same style as Samus'.
Ridley would get a pixel alt from the original Metroid as well - I'm aware of the reference alt, but it would be a nice touch.
Ganondorf, Zelda, Sheik and Young Link would all get sprites from Ocarina of Time.
Kirby gets a pixel sprite from the Gameboy, in the original colour, as does Dedede and Meta Knight.
Pit and Palutena get their sprites from Kid Icarus, on the NES.
Snake gets a Metal Gear Solid alt.
Donkey Kong, Diddy Kong and K.Rool all get 16 bit alts, in reference to Donkey Kong Country. Donkey Kong also gets an alt from the arcade game.
Sonic gets a 16 bit alt too.
Mega Man gets his 8 bit alt, but also a Mega Man X 16 bit alt.
Pacman gets a pixel alt from Pac-Land (I think it's called that?)
Duck Hunt Duo get an alt from the original game.
Ryu and Ken get their Street Fighter II 16 bit alts.
Simon gets his Castlevania I alt, as well as one from Super Castlevania IV.
Richter gets his Rondo of Blood alt.
Banjo and Kazooie get an alt from their N64 games.
Terry gets an alt from Fatal Fury, King of Fighter (his first game.)
Now, I know I probably forgot some but these were what I could think up off the top of my head.
Another idea is that characters would have say three or four selectable sections - I'll use Captain Falcon as an example. You could select the jumpsuit he's wearing, his helmet, his scarf, and his gloves and boots (as one entity) and each would have a colour wheel allowing people to make custom alts. I imagine people would get really creative with these - and it would, reduce the need for people to hack to get their favourite alts from previous games or fan favourites (Yes, Fire Mario, someone's gonna say it.) This would possibly also mean that some proposed echo fighters wouldn't be absolutely necessary - such as Zack Fair over Cloud, for example. Perhaps these alts could even be shared over Switch Online, akin to how Mii Fighters are shared? This would likely be a rather popular addition, but I personally think it should be in an update rather than as DLC.
It's not a leak or anything, just a suggestion - but what do y'all think?