More single-player stuff is possible. Maybe an Octoling mode (would make the tumblr post about Octolings make sense, at least to me) where you play as an Octoling and the enemies are Inklings with various weapons (and the final level is Cap'n Cuttlefish backed up by Agents 1, 2 and 3), or maybe just new levels. Or a Hard Mode, I'd like that. Or all of the above.
I saw Octoling amiibo in this thread, and that would be cool as well, though I don't see it happening any time in the near future (but I'd probably get it if it did). Perhaps we could get more amiibo functionality. Either just themed clothes (2-star stuff, like the other amiibo gear), or more challenge levels (like on the Splatoon amiibo).
And maybe playable Octolings in multiplayer modes. That's a possibility as well, and I wouldn't have a problem with that happening.
Also, more stages.