A mode that was present in the earlier Super Smash Bros. games but was cut from Super Smash Bros. Ultimate will soon be returning, as a new commercial for the game has revealed that the Stage Builder will be coming back in a future update.
Super Smash Bros. Ultimate boasts one of the biggest rosters in fighting game history, but the game was missing a few modes from the earlier titles when it was launched. One feature from the older games that was cut from Super Smash Bros. Ultimate is the Stage Builder, which allowed the player to create their own levels using pre-made assets. The Stage Builder was present in Super Smash Bros. Brawl and Super Smash Bros. for Wii U, but it did not return in Super Smash Bros. Ultimate.
It seems that the Stage Builder will be added to Super Smash Bros. Ultimate soon, as a new commercial for the game shows a brief glimpse of the "Games & More" menu, which now includes an icon for the Stage Builder mode which doesn't exist in the current iteration of the game.
The last Nintendo Direct promised that a huge update was coming to Super Smash Bros. Ultimate in the upcoming 3.0 patch that is due to arrive in the spring. There were no details given about the contents of the patch, though it's almost certain that it will add Joker from Persona 5 into the game as the first paid DLC character.
Super Smash Bros. Ultimate was released just in time for the 2018 holiday season, which led some fans to believe that the game had to cut features in order to meet the strict deadline. The fact that the game is missing the recurring Home-Run Contest mode is considered to be a sign of this, even though most of the assets for the mode (mostly the bat and the sandbag) are already present in the game.
It's possible that Masahiro Sakurai's team had to cut some planned content from Super Smash Bros. Ultimate in order to meet the holiday season deadline, with the intention of adding them back in later through updates. The fact that the Stage Builder mode is returning seems to be proof of this and it also means that other new content might be coming to the game alongside the new DLC characters.