https://www.gamespot.com/articles/e3-2018-super-smash-bros-ultimate-is-an-entirely-n/1100-6459758/

Is Super Smash Bros. Ultimate on Nintendo Switch a new game or just an upgraded version of the most recent Wii U/3DS installment? While it wasn't explicitly stated during Nintendo's E3 2018 Direct presentation, during a Q&A session following the reveal, Nintendo's Nate Bihldorff, a senior localization director, confirmed that this next Smash will be a "new game built specifically for Nintendo Switch."

It's not a port or an update, but an original title built from the ground-up for Switch. However, as to whether the game will include a single-player component remains to be seen. However, given the cinematic reveal trailer for Ridley, and the fact that there are a significant number of characters to unlock, it's likely that there'll be some kind of solo campaign.