Game of Thrones: Please Let This Shaggydog Conspiracy Be True
The lords of the North were all present when Robb Stark (Richard Madden) named Jon Snow as his heir shortly before the Red Wedding, but this proclamation hasn't been mentioned since.
Evidence against the theory: A casting notice for Season 6, as reported by Watchers on the Wall, sounds an awful lot like Wyman Manderly.
If the show does plan on introducing the Manderly plot, it wouldn't make sense to give away the meat of the conspiracy to the Umbers.