It looks like the recently-announced Super Smash Bros. Ultimate may have been teased several years ago via Super Smash Bros. for Wii U. This stems from an in-game mission called The Ultimate Battle, which is attached to the following description:
They’re all here! Every single fighter! If this battle’s not “ultimate”, then what is?
As you may have already heard, Super Smash Bros. Ultimate will include every single fighter from the series, pointing toward the possibility that director Masahiro Sakurai was already thinking of the next game.
Source: Source Gaming @AllSourceGaming