Sometimes, it is totally worth it to watch innocuous trailers for already existing games. Nintendo proved it with the latest trailer for Super Smash Bros. Ultimate. In the opening moments, a whole new feature was revealed for the game inadvertantly! It's Stage Builder!
If someone starts watching the Super Smash Bros. Ultimate "Nintendo Switch My Way" video, they'll see it starts with a kid who is grabbing the controller from an entertainment center where the Switch and a TV with the game playing is. The TV is showing the Games & More section of Super Smash Bros. Ultimate. However, the menu is different in this clip. Normally, you can find Classic Mode, Training, Mob Smash, Mii Fighters, amiibo, and a chance to fight characters you missed beating here. In this new video, an extra option appears next to amiibo: Stage Builder.
Nintendo has not said anything about Super Smash Bros Ultimate Stage Builder yet. It wouldn't be unusual for such a mode to appear. It is a Super Smash Bros. staple. The feature first appeared in Super Smash Bros. Brawl on the Wii. The Wii U version of Super Smash Bros. also offered it. People could draw platfrms, place items, use stage parts, and generally make all sorts of custom stages to use with friends. Though, in the Wii U version, you couldn't use them in 8-player Smash.
We'll have to wait and see what the Super Smash Bros. Ultimate version of Stage Builder will be like. Hopefully, it will have the ability to share the stages we make online. The Wii U version of the game allowed people to do that after an update. The version 3.0.0 update is supposed to be released for the Nintendo Switch in April 2019, so it is possible we could see this new feature very soon.