Okay, the general consensus is that Sansa will somehow gain control of the Vale, and of course, their fully intact army. But what will she do once she has this army?
What will her opening move be? Head North immediately to take it back, or start with the Riverlands? Head to the Wall and help Jon if she finds out about that whole mess? Maybe get revenge of Cersei in the south before heading North at all?
To kick things off, I'll say I believe she'll retake the Riverlands first. It'll be the first campaign of many, I'm sure. But, of course Cersei isn't going to lie down and take that. Cersei's power lies at court, however, and the most external support she gets from outside the Crownlands comes from the Westerlands, which she technically controlls at the moment. The strategic thing to do would be for Sansa to take the Westerlands with her now combined forces of the Vale and Riverlands, and install Lancel--who is conveniently seated at a holdfast in the Riverlands--as the Lord of Casterly Rock and use him as puppet ruler. Sansa then can force Lancel to disown Cersei from house Lannister and cut off their financial and military support and use it for her own.
With the combined might of three regions, shakey as her grip will be on all three of them, Sansa's next move would most logically be the Reach. She may try and take a crack at the Reach and cut off the other significant ally of the Crown, the Tyrells. However, I believe this could easily be solved through a secret alliance rather than actual warfare. In exchange for Margeary's safety and retaining power after Cersei's downfall, as well as perhaps some help with the Ironborn, the Tyrells agree to pretend to be fighting off Sansa's campaign through their region and losing against her. They allow her to pass through their territory with some "spoils of war" (food, weapons, and other supplies, as well as perhaps some troops in the guise of rebel turncloaks).
After passing through the Reach, Sansa's forces would probably head directly to King's Landing and lay a siege to the city. Or they could Trojan Horse it like Tywin Lannister, which would probably be smarter and take less time. The end game here would be Cersei's ultimate downfall and the Lannister's hold on the crown completely disappearing, but i won't go into detail on that.
After King's Landing is taken, Sansa will be, by all rights, the ruler of Westeros, but I doubt she would stay long in the city. After a few weeks, she and her forces would probably head immediately North to reclaim it. This would be far easier done now than if Sansa had tried to do so with just her Vale army and with Cersei most likely coming after her as soon as she surfaced. Now, Sansa has a much, much larger army, Cersei would be disposed of, and the Boltons would be totally screwed with the Iron Throne no longer backing them.
There might be conflict with Stannis in the North, and Sansa may eventually have to deal with the Ironborn as well, but I think the immediate threat of the Others would put those matters of the backburner for some time. When Daenerys shows up--if she ever does (insert eye-roll)--I don't think Sansa would be opposed to giving up the Iron Throne itself, but the North, Riverlands, and Vale. Hell no.
I think the conflict with Daenerys might be the final military/power conflict for Sansa in the story. Whoever wins is up in the air, considering all that could happen on Daenerys end hypothetically once she gets to Westeros. But I think it would like to think it ends with Sansa keeping the North, Riverlands, and Vale for herself, while Daerneys gets everything else, and the Westerlands, while not technically Sansa's, would remain in her political strangle hold thanks to Lancel.
What do you guys think will happen?