Originally Posted by
eschatological
This is the best part that no one has even bothered to think about. D&D's writing and planning have literally fucked over LF's master plan.
There is no way Sansa, with Brienne in her service, will ever trust LF again. LF can't ally with the Starks (and even if he did, then he literally betrays Cersei who he manipulated). He can't rally the Vale to fight against Starks because the Vale already hate the Lannisters for Jon Arryn's death and the Starks are their natural allies in that fight. He can't fight the Starks with some borrowed Lannister army or something and win the fight against the Starks, because the North will never accept a Southern Lord as their Warden. He has no one left to put in as a puppet in Winterfell. Cersei will know he's betrayed her if he does anything but march against a Stark army re-taking Winterfell. About all he can do, feasibly, is sit in the Vale and do nothing.
And of course, it all started because he married Sansa off to the Boltons, the ultimate dumb move. Knowing what he knew at the time, he should have seen that there was no way that benefited him.