my big concern about the identity of the next boss is in regards to the strawhats' bounties. they just got their big bounty increase after enies lobby and are now fighting a shichibukai, so if they fight another member of the three powers, it would definately call for another bounty boost, which i think would be too early. just because we now have the three powers named does not imply that they will be the only opponents from now on.
i'm hoping for another arc like skypiea with an exraordinarily strong opponent like Enel who is unknown to the gov't and thus won't count towards bounty. If it takes place in the underwater wonderland of Mermaid Island, we may even get the assistance of the friendly shichibukai, Jimbei that i've been hoping for