In the books Cat describes Littlefinger as "... always clever, even as a boy, but it is one thing to be clever and another to be wise."
I get the feeling that between dropping all of Sansa's post-ASoS material/development and rushing to the end of the series Littlefinger just sort of ran out of relevance to the writers as soon as he left KL. There was zero logic in his Season 5 plot and then they dropped the 'I do everything for Sansa' crap at the end of season 6, which goes directly against what he was doing in season 5, where him doing nothing would have gotten him closer to that goal than marrying Sansa off.
Had they'd seeded the Ramsay/Sansa marriage before they started writing season 5 (since they claim to have had the idea since season 2) it should have played out the same way the fArya/Ramsay marriage does in the books, with Littlefinger convincing Cersei it was in the best interests of the Lannisters to strengthen their Bolton allies with a trueborn Stark and then had Littlefinger stay in King's Landing for seasons 4/5/6 and play a part in Cersei's story. Replace Qyburn as the one whispering bad advice in her ear and let him take the place of Varys from ADWD in a blatant move to replicate Tyrion's crossbow killing of Tywin and push Cersei even further, which ties up nicely with both Varys (his opposition since season 1) and Tyrion (a character well aware of how duplicitous Littlefinger really is) both turning up with Dany to take King's Landing in season 7. Obviously not perfect and would require a lot more fine tuning to make it work but it would have been better than what we got.
I'm really not looking forward to them trying to paint him as a political genius in season 7 just so Sansa can look like she 'outplayed' him when it's time for him to die. Not a spoiler btw, before anyone panicks. Just a prediction.
I think Littlefinger is in as interesting a spot as he's ever been right now. Since the beginning of the series, he's managed a clear upward progression, from brothel owner, to master of coin, to lord of Harrenhall, to presumably soon-to-be Lord of the Vale.
Delivering Sansa to the Boltons and then informing the Lannisters was supposed to be the next step in that trajectory - if the Lannisters won the resulting conflict, Littlefinger would be the obvious choice for Warden in the North, but if the Boltons won, he'd have solidified his northern alliance and the political landscape in the south would be wide open. Maybe he could've even taken on a weakened House Bolton with the knights of the Vale and 'rescued' Sansa to claim the north.
But, all that depended on Sansa being on his side. When he whiffed on his read of Ramsay Bolton, he ruined pretty much all of his plans. Now Jon Snow is back in town, Sansa despises him, and he's officially, outwardly aligned himself against the Lannisters by siding with Winterfell. For the first time in a while, maybe that we've ever seen, his plans are super fucked and his back's against the wall. I think that has plenty of potential for him to have an interesting story this season, if it's written well.
