But if Vergil got yamato back, and took over Nero, as purported, then wouldn't he end up with the same amount of power that he had previously whilst wielding the same sword? Where would the extra power come from? And think on it: Nero's arm is the source of the Devil Bringer, right? Well then, what would happen if the whole of Nero became like the arm? That's some strange DB!
It seems that the only way that Vergil could get more power is to either try to gain his amulet again or... to steal the sparda sword from wherever Dante lets it end up. Unless, of course, they add in other humanoid demons, like Sparda, but not as great... even then it would be interesting... Either way, it would take a new introduction of demons to bring another source of ultimate power to Vergil - if he's even alive. And, if he isn't alive then... At least we still have Dante, right?!