I think they're setting Jaina up to replace Wrynn by making her more vicious and unstable (aka the old him, needed to keep the base HvA conflict fueled).
The last stage of Isle of Thunder really seemed like the "new Horde and Alliance" meeting. That being said, I think they're making Vol'jin a decent contender too. Possibly a co-warchief situation? Especially after seeing what one tyrant can do?
My prediction: We fight Garrosh to near-death. It's revealed he ate a Sha/Old God/whatever to gain it's power or something, and we fight the Sha. The only way to finally "kill it" is for Varian to sacrifice himself, getting his moment in the sun and full-circle redemption. Lormethor and/or Vol'jin step up and fully articulate that this became their plan long ago. Jaina blames them for allowing Garrosh to 'get this far' as part of the plan and steps up in Varian's place (possibly another Bolvar to Anduin or Anduin is just lost to the land of Pandas and meditation), thus keeping the base conflict going despite the war seemingly coming to a head. Also keeps the faction losses "fair" (sort of).