At this point, Petyr Baelish is going through allies as quickly as Aidan Gillen goes through accents (listen as Baelish becomes increasingly more leprechaunish over the course of four seasons). It’s unclear if Lord Baelish is kind of good but mostly evil, mostly evil but kind of good, or just a huge asshole. It depends on what you think he's up to.
Baelish has been behind so many curtains you'd be forgiven for forgetting that he lied about the origins of Tyrion Lannister's dagger, introduced Ned Stark to one of Robert Baratheon's bastard children, and orchestrated the betrayal of the City Watch that led to Ned's death — all in season one. In other words, Sansa has a lot to be worried about. But maybe not as much as you'd think.
Olenna Tyrell, if not exactly the Danny Glover to his Mel Gibson, is the closest thing Baelish has to a partner. They are the Batman and Robin of screwing over the Lannisters - though the Queen of Thorns Petyr as more of a primped-up vulture. But Baelish doesn't need effection when he's got leverage in the form of "a handsome young man."
Riddles and oblique references are par for the Baelish course, so the identity of said pretty boy is wide open to interpretation. One plausible candidate is Lancel, who spilled Cersei’s beans to the High Sparrow, but that raises the question of how Baelish could have been pulling a zealots strings from all the way up north. Another option, perhaps the most likely, is Baelish’s spy Olyvar, who betrayed Loras to escape his own punishment. Olenna would be happy to have that young man either recant his testimony or choke to death on his own blood. She’s equal opportunity that way.
The options don't end there: Gendry is still in the mix (it wouldn't be the first time Baelish used him as a chip) and Robin Arryn is going to knight school with Lord Yohn Royce at Runestone. The best revenge is to live well and the Vale would suit the Tyrell's rather nicely, allowing them to consolidate gains closer to the capital.
Our Theory: Whatever Baelish's plan may have been prior to Cersei's incarceration, he'll want to speed it up now, taking advantage of the Lannisters' weakness while shoring up support from families that increasingly rather than decreasingly powerful. That doesn't bode very well for either Lancel or Olyvar. Hurting Lancel compounds the Lannister problems while convincing Olyvar to recant will get him thrown in jail and Queen Margery removed to rule the Seven Kingdom with a golden fist.