- Sansa marries Ramsay.
- Littlefinger narcs on the Boltons and pretends like the marriage plot was entirely their idea.
- Cersei gives him permission to go to the North with Vale knights to “punish the Bolton traitors.”
- Baelish gets his Vale knights, heads to the North, fights and defeats the Boltons.
- Baelish installs Sansa as the Lady of Winterfell. She’d have support from most northern houses and the Lannisters wouldn’t be in any position to challenge this (which Littlefinger ensured by getting both the Lannisters and Tyrells entangled with the High Sparrow).
- Baelish rules the North through Sansa the way he rules the Vale through Robert Arryn. He’s also the lord paramount of the Riverlands in exchange for brokering the Tyrell alliance. Baelish now rules the three regions whose paramount families, he feels, screwed him over.