Game of Thrones spoilers below, duh.
After Game of Thrones episode seven, "The Broken Man," a Photoshop master deciphered Sansa's secret letter, confirming its fairly obvious destination and message: "Littlefinger" and "help me with your big army," respectively.
Now, the robust subreddit r/Gameofthrones has another opinion on the ongoing chain mail saga in the North. From a user named ajxnjd:
This is about Sansa's letter to LF and why she sent it.
I think I cracked this up. Remember when Sansa insisted in sending Brienne and not sending a raven to ask her uncle for help? Her argument was that she didn't want Ramsey to intercept the Raven and knows about this. But now it seems like she's gonna send a raven to ask little finget for help. But why? Shouldn't she be afraid that Ramsay will find out? Well here's my theory. Sansa wants Ramsay to find out about little finger army. Ramsay, being the mad dog that he is, will send a good part of his forces to deal with LF and knights of the vale. Now, Sansa and Co will fight a reduced Ramsay army due to this and they win easily.
TL_DR: Sansa will screw LF over to win Winterfell.
This is an intriguing theory, especially because it gives Sansa even more credit in her development from Joffrey's Naive Fiance to Littlefinger Lite. She's expecting Ramsay to intercept her letter, divert his army to handle it, and allow Jon and her to take Winterfell with little resistance.
Of course, there are some flaws to this theory, and Reddit was happy to point them out. For one, commenters said that Littlefinger's army would be much too large and well-trained for Ramsay's. Others thought it was actually a good plan, but it wouldn't work against Ramsay. He holds Moat Cailin, so he'd destroy any army coming up the Neck. Gambitsun doesn't think Ramsay would take the bait in light of the odds:
But BlueAndOrange92 brings up perhaps the most intriguing tie-in of all:
The Neck is home to the crannogmen, ruled by Howland Reed. Fiercly loyal to the starks and they can actually bring forces through the swamps without needing Moat Calin. It's why Robb sent Maege Mormont through the swamps when she had to go north
They're also great at poison darts and guerilla warfare. If Ramsay left a minimal force then they're probably already dead
Crannogmen are loyal, and you can't invade their lands to push them out because even their castles move through the swamps.
When Moat Calin isn't actively being held it defaults to the Reeds
Show-watchers and book readers alike have been waiting for Howland Reed to finally make an appearance for years. If Littlefinger brought his army to Winterfell with the help of Reed, fans would lose their minds.
But the show's largely avoided the realization of any complicated theories, adhering mainly to Occam's Razor. Jon Snow didn't warg into Ghost before being brought back to life, a long-held fan theory; Melisandre just revived him. Arya wasn't secretly the same person as the Waif.
Plus, the upcoming attack on Winterfell in episode nine is supposed to be the show's largest battle to date. It would be significantly smaller if the troops of the Vale never make it there. Most likely, they'll arrive in the nick of time to help Jon and the Wildlings take Winterfell without any part of this theory coming true.
Still, the hype won't die. And even if all your theories are discredited in the show, you'll always have the books.