Do you believe in the great northern conspiracy theory? Because I think that's what happens in the end based on S6 conclusion. The northern lords together declare Jon as KiTN. Only with Robb's will to validate stuff..
I think it’s partly true. There are pockets of resistance to the Boltons throughout the North, but it’s unlikely they were coordinated into one grand conspiracy before Roose summoned them all to Winterfell for Ramsay’s wedding. The North is enormous, and to coordinate by mail they’d have to trust every maester in the region not to be a spy.
My general sense of it is that there’s a Small-to-Middling Northeastern Conspiracy, led by Wyman Manderly and backing Rickon, and a Small-to-Middling Northwestern Conspiracy led by Maege Mormont and backing Jon. (It’s not impossible that some dissident Riverlanders and former members of the BWB have formed their own faction or joined up with the northwestern group, though I’m less sold on that.)
Both groups have the same primary objective of crushing the Boltons. They have diverging secondary objectives and emotional investments - the northwesterners are explicitly nationalist, while the northeasterners really want to make the Boltons and Lannisters pay. Their respective claimants do make sense: honoring the Young Wolf’s will is its own statement of Northern legitimacy for the nationalists, while sticking to conventional succession and declaring for Stannis shows Manderly’s faction looking south. That reflects the broader cultural diversity of the North, in that White Harbor and other towns to the east or south of Winterfell are more Andalized, while the farther-flung rural areas keep to the older way.
That said, I doubt the different factions are opposed to each other or unwilling to work together. I think the different approaches are more reflections of cultural preferences and limited information. Thenortheasterners were loyal to Robb, and the northwesterners are with Stannis now. (He really should lay off the heart trees, though, that crowd doesn’t care enough about him to tolerate disrespect to the old gods.) They mostly care about dealing with the Boltons.
There wasn’t a grand northern conspiracy before, but there sure is now:
"I have just come from the high table," Lord Wyman went on. "I have eaten too much, as ever, and all White Harbor knows my bowels are bad. My friends of Frey will not question a lengthy visit to the privy, we hope." He turned his cup over. "There. You will drink and I will not. Sit. Time is short, and there is much we need to say.” (ADWD, Davos IV)
"We must look at Manderly," muttered Ser Aenys Frey. "Lord Wyman loves us not."
Ryswell was not convinced. "He loves his steaks and chops and meat pies, though. Prowling the castle by dark would require him to leave the table. The only time he does that is when he seeks the privy for one of his hourlong squats." (ADWD, A Ghost in Winterfell)
The people inside Winterfell are mostly older men on that final hunt, to save food for their families and bring the Boltons down with them. Lord Wyman will put together whatever seems most likely to work, and those of us who are following the show have a pretty good guess what that’ll be.