The director of the Super Smash Bros. series has spoken out about Super Smash Bros. Ultimate's DLC content, as well as the next Super Smash Bros. game.
Masahiro Sakurai recently took to Japanese magazine Famitsu to discuss the coming six DLC fighters to Super Smash Bros. Ultimate, and to talk about the next Smash Bros. game after Ultimate. Sakurai says that all of the DLC fighters were chosen by Nintendo and that while working on Super Smash Bros. Ultimate, he can't direct any new game idea. Sakurai also mentions that he is ok with this, as he wants to work to make other people happy, and not himself.
Sakurai also touches on if there is a new Super Smash Bros. game in development already. Sakurai says that at the moment, Nintendo currently doesn't have any plans for a new entry in the Super Smash Bros. series and that he doesn't know what will happen with the franchise yet. It's highly unlikely that Ultimate will be the last entry in the Super Smash Bros. series, especially when you consider that Ultimate is the best-selling game out of the entire series, and out of every fighting game in US history.
Nintendo is most likely planning the next Super Smash Bros. game for their next big console release, as they will want Super Smash Bros. to be their flagship title that entices gamers to pick up the new console. Nintendo also began development on the next Super Smash Bros. game right after they completed all the DLC content for Super Smash Bros. for Nintendo 3DS and Wii U.
So, taking into account how successful Ultimate is and how long a new Super Smash Bros. game takes to develop, I would say that Nintendo is most likely doing the same thing for Ultimate. Once the six DLC fighters are released, development for the next Super Smash Bros. game will begin for Nintendo's next console.