Game Of Thrones: 10 Most Believable Fan Theories
4. The Great Northern Conspiracy
Robb Stark's death at the Red Wedding left a power vacuum in the North, one without a Stark to fill it. So it is that the Boltons have now taken over Winterfell, with Roose installed as Warden of the North and the backing of the Lannisters behind him. However, there's one key point missing: The North Remembers.
So, the Northern Conspiracy theory goes that before his death, Robb made a will that legitimised Jon and named him as his heir. The Northern Lords have discovered this, and combined with outrage over Robb's death, are now plotting to see this come to fruition and raise Jon to be Lord of Winterfell and Warden of the North. This especially makes sense given that Bran and Rickon are presumed dead, Arya is supposedly marrying Ramsay, and Sansa is married to Tyrion (or, in the show, Ramsay, with Arya also thought dead).
A huge number of Lords have thus conspired to push Ramsay and Stannis into war, leaving one dead and one weakened, so they can then swoop in and claim the North for Jon. This very much likely includes the Greatjon, who judging by a casting call could be returning next year, and Lord Wyman Manderly, aka Lord Too-Fat-to-Sit-a-Horse, who could also be appearing in some form in season 6.
Manderly is particularly important, since he knows Rickon is still alive, and has sent Davos to retrieve him. This could be as backup if the Jon plan fails, or if Jon becomes King of the North, Rickon may serve as Lord of Winterfell. The seeming refocus of the show on the North makes this theory even likelier.