Honestly, at this point even I'm not sure what Littlefinger's endgame is. Assuming he's telling the truth to Sansa, his immediate plan is to create that North-Riverlands-Vale alliance/merge. However, I'm not sure if he'd be content with those three, or would instead just keep going and work on taking over the remaining Kingdoms. As far as I could tell, his original plan was nothing more than "Get Ned exiled so he could get Catelyn" and once Joffrey completely derailed that he's just readjusted it to "Hand Sansa either half the kingdom or more and win Sansa in the process."
The other thing is, I have to assume that the second Littlefinger goes through with this plan, basically everyone else in Westeros is going to be after him. Stannis obviously isn't going to settle for the Seven Kingdoms being divided (at best), and I doubt Aegon, Daenerys, or Cersei would either. Thing is, once Littlefinger plays that card, I think he basically burns his bridges with everyone in the South. Even to this moment King's Landing sees him as an ally. Both Cersei and Kevan have internally thought to themselves that who they really need back on the council is Littlefinger. Hard to imagine that kind of thinking keeps up once he ends up taking half the kingdom with him.
Of course, he could tell Sansa to only act like she's allied with the Vale, while he pretends the Riverlands are still loyal, and continues to cause the South to implode. Varys seems to be doing a good job at that though. Alternatively, Littlefinger only cared about the Riverlands to the extent that being Lord Paramount allowed him to gain control of the Vale. Afterall, the Riverlands should be the hardest to defend of the 3 I would think, he could just sacrifice the Riverlands and keep the 2 much more defensible locations. Honestly, I wouldn't be surprised if by the time Daenerys arrives, it's Littlefinger and Sansa (and possibly Aegon before them) who she has to depose to unite the Seven Kingdoms.
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xp1337: Don't you wish there was a spell-checker that told you when you a word out?
GotD (219/384) Melee v FFX