Masahiro Sakurai revealed during the final Direct for Super Smash Bros. Ultimate that DLC would be coming to the game in the form of six characters, with the first being Pirahna Plant from the Super Mario Bros. series (who would be free to those who bought the game at launch) and the second being Joker from Persona 5.
It seems that more DLC is planned for the future of Super Smash Bros. Ultimate, as the Nintendo of America support page concerning the DLC for the game has added a line about costumes for the Mii Fighters that will be purchasable in the future.
The Super Smash Bros. Ultimate DLC support page breaks down the prices of the Fighters Pass and the individual costs of each upcoming character, as well as mentioning that Mii Fighter Costumes will be coming in 2019 and will retail at $0.75 each.
There technically already is a paid DLC costume for the Mii Swordfighter, as the only way you can use a costume based on Rex from Xenoblade Chronicles 2 is to purchase the Fighters Pass. It's possible that the Rex costume may be one of the ones on offer in the future.
Super Smash Bros. 3DS/Wii U had quite a few DLC costumes on offer for the Mii Fighters, so it's no surprise to see them returning for Ultimate, as aesthetic customization is the whole point of playing them.
The Mii Fighters are likely to see a lot more play in Super Smash Bros. Ultimate, as the ban on using them in the online mode has been lifted, which means that we may see a lot more costume DLC on offer than last time.
The most surprising aspect of this reveal is that the developers of Super Smash Bros. Ultimate aren't making costumes for the other characters, as the Mii Fighters have never been that popular compared to the rest of the cast. You would think that costume DLC for the likes of Cloud and Link would sell way more than those for the Mii Fighters.
The most important aspect of this reveal is that more DLC is in development outside of the Fighters Pass, which means that costume DLC for the other characters isn't off the table yet.
We may also see more characters being developed in the future, with a second Fighters Pass being a possibility somewhere down the line.