"Who rules East Europe commands the Heartland;
who rules the Heartland commands the World-Island;
who rules the World-Island controls the world.”
(Mackinder, Democratic Ideals and Reality, p. 106)
Now, we know that the Riverlands are the Heartland (also called the “pivot area”) of Westeros, (the “World-Island”). GRRM has told us as much several times over by describing how the Riverlands have been invaded and fought over for centuries, only achieving stability once Westeros, the “World-Island”, had been conquered. Now, the Riverlands are also crucial to Westeros’ geopolitics for another reason: it contains one of the biggest choke points to the North—The Twins.
So what does this mean and why am I talking about dots on a map?
Because Sansa Stark is going to (for reasons besides narrative) going to be the Stark to take back Winterfell (at least), even if she gives up the seat once Bran or Rickon come back from the far North.
She’s not the closest Stark to Winterfell, but she’s the Stark closest to the pivot areas—the primary one of the Riverlands, and the North’s smaller pivot area, locked in between Moat Cailin (a choke point), White Harbor (main source of trade, resources, and wealth for the North), and Winterfell/Winter Town.
Not only is Sansa the closest to these points, she is also located (hence the ‘geo’ in geopolitics) the most strategically out of the Stark siblings. She’s in the Vale, which has an affinity for both House Stark, since Ned was raised there, and House Tully, because of Lysa and Blackfish. If Littlefinger’s plans come to fruition, she marries the heir and very shortly becomes Lady of the Vale.
She’s the closest member of House Tully to the Riverlands. With Edmure in captivity and his heir in captivity, Sansa, should the Riverlands rise for House Tully (which may be in Littlefinger’s plans), Sansa becomes de facto head of House Tully as the only visible heir of Catelyn’s line. And by staying in the South, as of yet, Sansa avoids falling prey to the descending White Walkers. Greyscale, however, will soon become an issue.
She’d be able to overtake the choke point of The Twins if she can win the affinity of the Reeds and storm the bridges from both sides. The Reeds will lead her forces through Greywater Watch and she can meet with the Manderlys and their forces in the Heartland of the North and march to Winterfell.
Sansa is the Stark (barring a reappearance of Arya) who is best located for a reconquest. Most southerners consider her the heir apparent to the North. She has a political base in the Tullys and in the Arryns as well, and Littlefinger. She has the access to the most resources. She has access to the Heartlands. There is currently very little threat of White Walkers.
For the record, I have no opposition to Sansa giving up her seat to Bran or Rickon when they come back. But as it stands, Rickon needs someone to get him off of Skagos, and it’s not going to be the Skagosi, and his other hope is Davos. Bran, when he reappears, can align himself with Stannis and Jon at the Wall (if Jon is reanimated by Melisandre), but the Wall is a very volatile place at the moment. I’m of the opinion, though, that Winterfell can be taken from the south, since as of now most of the north has realized that their resources are going to be needed for winter and the oncoming magical fight, while the south remains focused on politics, and I firmly believe that Sansa is in the south for that reason. She can also take back Winterfell and the North in the name of House Stark without being the head of House Stark.