So, I actually posted in a comment here earlier about how I'd like to see the alternate costume system revamped in a future
Smash title, and I think it'd be fun to actually delve into that a little and how it would work. Firstly, credit to the
Super Smash Bros. Historia fan project for the basic inspiration behind this - I've added my own twists, but I think their project is hella fun and you should check it out!
Let's take Mario as our example.
Each character now has two "bars", consisting of
Colours and
Costumes.
Colours are just that - recolours of costumes that allow for different references to be made without a whole new costume of their own. Check out some of the examples I've added! Each of them should, if possible, be a direct reference to their source material in some way or another - and, in some cases, unique textures might apply (such as a Cosmic Mario colour for his default look.) Each Costume features a minimum of eight Colours, though some may have more than others.
Comparatively,
Costumes are model-swaps based on a character's other designs or appearances, though they don't change the moveset or skeleton of the character. However, they can feature different cosmetic features - anything from idle animations, to taunts, to voicelines, the appearances of some attacks, or even announcer lines if we're talking about representing an entirely different character (like Olimar's Alph, or Hero's Erdrick/Solo/Eight!)
With this in mind, I think these elements make for additional single-player content pretty naturally. Let's say, in the base game, each character has four Costumes of which two are unlocked from the beginning and you need to unlock the other two by playing the game - plus, you could add Costume DLC down the line to include more costumes overall and, well, make more money, whilst worrying less about outright development time.
Having costumes handled like this can make your roster feel like it represents
more in general without having to have more outright characters. For instance, Mario immediately springs to mind as having a lot of potential for representation - say, if he lost his F.L.U.D.D., he could still have a
Sunshine alt of some kind, or Dr. Mario being brought in as an alternate costume instead! On top of this, it could work as a really creative way of showcasing Nintendo's history. One of my personal picks for Samus, for instance, would be a "retro" alt based on her appearance in
Nintendo Power #29 and the manual for
Metroid!