Littlefinger is, much like Varys, inciting the various great houses to destroy each other while accruing more and more influence for himself. His various dealings are all aimed at drawing factions into the conflict, then weakening/undercutting them.
While I expect he still has a few cards close to his chest, the plan he details to Sansa at the end of AFFC makes plenty of sense. With the Lannister-Tyrell alliance falling the pieces, Dorne presumably to enter the fray soon along with Aegon, etc., he has at this call the only fresh and unbattered army in all of Westeros (the Vale knights). Marrying Sansa to Harry Hardyng creates a powerful alliance under his control that ideally brings the North (weary as it is), the Riverlands (between Littlefinger's notional lordship and the more important loyalty to House Tully, as represented by Sansa) and the Vale. With that he could rule the whole place (either by becoming king himself or, more likely in my view, using Harry and Sansa as his figureheads, since he seems to prefer smartly staying out of the limelight and holding the real power).
The question marks in this scheme are Dany (though he seems to be aware that she'll show up eventually, based on one comment) and the Others (which nobody is really taking into account, apart from Stannis and Night Watch).