Okay, so clearly Littlefinger is playing the game to acquire as much power as possible.
Obviously, removing Joffrey from the throne and simultaneously making the Tyrell's indebted to him improves his position quite a bit. Assuming Margaery does what she does and secures Tommen for a hubby, that puts the Tyrells in great position to reward him, and a stable royal pair on the throne to keep those rewards in place. All well and good.
But how exactly does Sansa fit into this? For one thing, he's apparently made a connection with Lysa. Now, of course, she's nuts and not exactly easy on the eyes, but if he has the inside track for Warden of the East, I'm not sure that forming a union with Sansa improves his position. Even if he believes that Bran and Rickon are dead, marrying Sansa would only give him dubious and relatively impractical claim as Warden of the North. After all, the north is currently divided between the Greyjoys and Boltons, and neither is likely to take their armies off the field just because Littlefinger rides up with Sansa on horseback requesting they honor the Stark name.
So, at this point does Littlefinger take Sansa to the Tyrells so they can make a union with her family name? Unlike Littlefinger, the Tyrell's DO have the power to quiet the civil war in the north and re-establish the Stark house up there with Sansa and, say, the Knight of Flowers. Littlefinger could deliver Sansa to Highgarden and absolutely secure himself as the future Warden of the East. But I suspect his ambition doesn't stop there. So what does he do with Sansa, and how does she further his ambitions?