Wow, this comes as a bit of a surprise. According to general director Hino Akihiro, Ni no Kuni II: Revenant Kingdom will have some form of multiplayer. In an official Bandai Namco interview video, several fan questions are put before Akihiro, and one of them asks whether there'll be any kind of multiplayer component.
"Yes, there will be multiplayer," Akihiro reveals. He then goes into a little more detail without really giving anything away: "You will be able to play online with your friends in various capacities." Interesting.
Obviously, this leaves us wondering what the heck it could actually be. Maybe there'll be a specific co-op mode that you can invite your buddies to? Minigames or something like that? Perhaps you'll be able to team up and take on waves of enemies? We've got no idea, but we imagine that we won't have to wait too long until more information heads our way.