I think some single player content would be very hype, as much as it can be fun to choreograph fights, or find a friend to play with, I do believe there is still room for solo content.

For example, if you add ai (No small feat I get that), an "arcade mode" wouldn't need to be much more than going through the list of the character select screen. Or break the targets might be fun to try in this game. Or a challenge/puzzle mode, where you are put in a sample situation and need to achieve various objectives.

If you want to be ambitious a "rogue lite" game mode, where you explore a location slaying enemies, and trying to find the boss, would be truly epic. Especially if you added dialogue (would be fine with "just" text) and a story to it.

I do not mean to say this is easy, nor am I demanding this for free, but even if you charged extra for a single player expansion, that would bring in a large group of people that aren't primarily interested in multiplayer.

I think this game adds a fine layer of polish to a rather new genre and has a chance to be something big, I really want to see this game grow. And I truly believe that single player content will be a big boost to this games possible audience.