My hope/prediction (I've read a lot of this in other posts so most of it is not my ideas, just stuff I agree with):
The Manderlys somehow alert Stannis that they are going to turn on the Freys, and together they destroy the Frey army marching on Stannis' position. The Manderlys return to Winterfell "victorious" and state that Stannis' army was smashed and present the Boltons with Lightbringer. The pink letter is legit, but the information is incorrect. The Manderlys inside Winterfell open the doors to Stannis, who sacks the city. Ramsay and Roose are either killed or executed, although I could see them possibly escaping only to be killed later by Northerners they thought were friends but were secretly loyal to the Starks.
Meanwhile, back at the Wall Mel sees the contents of the pink letter and, believing them to true, sacrifices Shireen to bring back Stannis. But Jon Snow is resurrected instead, so Mel then believes he is AA Reborn instead of Stannis. Stannis sends for Mel, Shireen and the wife to come to Winterfell, or perhaps he returns to Castle Black. Upon learning of Shireen's burning, he kills Mel (I'm thinking he strangles her with his bare hands) but is killed in turn by Selyse in the heat of the moment. The "Queen's Men" rally to Jon Snow becuase they learn of his resurrection, while the North unites behind the newly returned Rickon Stark (who would just be a figurehead for Wyman Manderly provided he survives the battle at Winterfell).