Super Smash Bros Ultimate has a large roster, even before wave 1 of DLC released and before Wave 2 even has confirmed characters, I have written before about many characters who could come to the game as DLC, but there is another possibility to consider as well. The mii outfits bring in the ability to customize the mii characters and dress them up as various characters, but the game could go much further than just leaving these as costumes for mii fighters.
To explain what I mean, let us take a look at Street Fighter V. Capcom’s latest Street Fighter game has a large amount of DLC costumes for the various fighters to wear, many of which reference other Capcom games. Some of these games referenced include Darkstalkers and Mega Man for instance and this is something the developers should look into doing for Smash. This can be done in one of two ways, the first being costumes the characters could wear. For instance, Samus could have powered armor skin that makes her resemble other characters including perhaps a certain highly requested Microsoft character, and Greninja could have a Strider outfit, considering how many fans feel there is a similarity between how Greninja plays and how Strider plays in the Vs games. Luigi could have a Mr L skin, while Zelda could have a Princess Hilda skin.
The other possibility is to have alternate skins altogether that would allow you to effectively have additional characters ala echo fighters. An example of this, would be giving Terry skins from other SNK fighting games, while Ryu and Ken could have additional skins for characters like Akuma. Link could in turn have a skin based on characters like Adol Christin or Takamaru, while Villager could actually become Balloon Fighter. Provided a character is a good fit, this could be a way to bring in many fan requested characters such as Shantae, while seeing how well these outfits sell to determine if the actual character might do well in a future Smash game.
Obviously this is something that requires a lot of thought before implementing, but would be a great way to sell more DLC and gauge fan interest in characters. This would require a lot of work still, but not at the level of designing new characters. It is something that Sakurai and Bandai Namco should consider doing in the future.