Definitely not base game.
Ultimate's project plan was done in December 2015, N Sane Trilogy was announced in E3 2016.
And while his iconic status could help a lot, he still wasn't relevant much by the time Ultimate started to be in the works and Sakurai had all the roster in mind.
So, unless there's some insane situation where Sakurai actually got intel from Activision before 2016, I'm not seeing it happen.
What I'm saying is, don't be dissappointed if he's not in the base game. DLC is a possibility though.