Seriously, the kind of battles that could go on would mimic the intensity of a pokemon duel. This could boost not only the sales of the game but the popularity tremendously. Maybe even Team battles of 5v5 or 3v3s orrrr even an RTS style Battle ground would just make this game an instant classic more than it already is... epic. I mean I'm still getting over how amazing this game is, but the missing online battles is what the next step needs to be in terms of DLCs.. not to mention people would pay for more DLCs introducing more familiars. The high reviews say enough, make it competitive online and it's a goldmine for them.
Equipping Ni no Kuni with downloadable content that would enable players to battle each other online... I don't think I would want to see that happening. Already Ni no Kuni has the Solosseum Series for pitting your familiars against a toughened array of opponents. Latching on a multiplayer game months after the release would probably not be a good idea in the sense that if people have to pay for it, what would be the point? Does spending $60 on Call of Duty's single-player outing, and then another $5-10 on its multiplayer mode after the game has already released sound appealing?