Cosmic77
Smash Hero
It'd be difficult, but I think it's doable. Just depends on how long they take to make the character. If they buckle down after E3 and milk the deadline up to the last quarter of 2021, I think they could make it happen.A year ago, I would've said to keep an eye on BotW2 for newcomer options. But now it's looking increasingly likely that it won't be out until 2022, putting it too late to safely pull a newcomer from. Much less a stage, Spirit Board, or music. It'd all have still been a work in progress when development on Fighter #11 started.
The stage would be the most difficult part, and we already have examples of stages being made almost a year before the game it was taken from launched (Wooly World). The character can go either way. If the character fights mostly with a weapon, then a majority of the moveset should design itself. It also wouldn't be as difficult if said character was also in the orginal BotW. Spirits and music tracks shouldn't require much time; I can't imagine either posing a problem to Sakurai, even if they started just now.
We'll see what happens though. It'll require a lot of cooperation between the teams of BotW and Ultimate.