Super Smash Bros. Ultimate has been in the works for good while now. In fact, it is possible that it may have been in development for so long that Sakurai might have teased the game in Super Smash Bros. For Wii U. In this game, there is a co-op event called “The Ultimate Battle”. The description reads “they’re all here! Every single fighter! If this battle’s not ‘ultimate’, then what is”? Of course, as Source Gaming pointed out in the tweet linked down below, it could be nothing more than a coincidence or an unintentional pun. Whatever the reason, all that can be done for the time being is to wait for an official explanation.
In Japanese the event is called, "究極決戦" which translates to "The Ultimate Battle" in English.
— Source Gaming (@AllSourceGaming) June 23, 2018