Please be Adventure or Subspace Emissary…
As part of the Super Smash Bros. Ultimate broadcast this morning, series creator Masahiro Sakurai talked about a lot of the game modes that will be in the game, including a lot of variations on the traditional fighting modes the series is known for (I say traditional, but nothing about fights in Smash is traditional). However, Sakurai then proceeded to show off the main menu screen for the game, with one game mode totally blurred out, teasing that it is something that he will talk about on a later date.
As for what this mode could be, that’s anyone’s guess. So far, no major single player component for the game has been announced, so it is possible this is that- it could be the long awaited return of Subspace Emissary or Adventure modes, both of which fans have been asking for for a while; Sakurai is making this the ultimate fan service game, so it is possible he brings one of those back (and the elaborate cutscenes used to introduce each new character hint at some overarching story mode).
Then again, it could be something stupid like Smash Tour or Smash Run again.