Hmmm, consider this about the Klingons. We're told in XI than an armada of 47 Klingon ships was destroyed by the Nerada; now I don't know about what scale the fleets are going to be numberwise, but that's gotta be a fairly significant chunk. Maybe in the next movie our new antagonist, whatever race, is going to try and take advantage of both the hits the Federation and Klingons have taken and move in.
That way the Klingons could make an appearance, but as an allied force, though maybe not necessarily friendly, instead of the villain of the piece, as both they and Starfleet try and deal with the new threat. On a side note, fleet actions could even have cameos from Starfleet ships by the name of
Kumari, or even a
Reliant commanded by Captain Carol Marcus
Who the new threat would be? Could easily be a TOS villain, though I'll admit part of me is tempted to see the appearance of the Cardassians

They've nearly always been shown to be Magnificent Bastards, and such characters have gotten favourable responses from audiences in some recent films. Heath Ledger's Joker in
Dark Knight comes to mind, though I don't think the Cardassians would need to go the crazy route.