One of the unique things about the Mii Fighters in Super Smash Bros. Ultimate is the ability to dress them up in various costumes, often mimicking characters that aren’t actually on the playable roster. As such, it should be no surprise then that they’re getting some DLC costumes in Super Smash Bros. Ultimate in 2019.
While Nintendo hasn’t highlighted this, the listing appears in their support page for Super Smash Bros. Ultimate. Specifically, it states that Mii Fighter costumes are coming to the game sometime in 2019, and each costume will be sold for $0.75 each.
With Super Smash Bros. Ultimate, several of the Mii Fighter costumes from the previous game — including those based on third party properties — were taken out. With this, it’s highly likely that these costumes, as well as some new ones, will be coming into the game as DLC.