According to the Nintendo support, Super Smash Bros Ultimate will receive more paid DLC in the form of Mii Fighter costumes, just how Smash 4 did. Each costume will cost you $0.75 (USD) and they will be releasing some time in 2019.
No other information is yet available, meaning we have no idea what the costumes could be. It is also yet to be officially announced by Nintendo, however since it is on the official Nintendo support website it probably wont be to long until we get that announcement and more information.
Source: Nintendo Support found through Nintendo Life
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