Lyanna Stark is Jon's Mommy
R + L = J
The underlying theory which everyone is hoping is going to be true when the credits roll on the final episode of the final season of Game of Thrones on HBO. Although there are thousands of theories by thousands of fans all over the web. There is no tracking them! Everyday someone interprets some fine print and pushes out another theory.
Lyanna Stark is Jon’s Mommy.
Why do I say that?
I've seen some theories around the books and the show though I haven't read the books. I'm basing my hypotheses on the interpretation of the David Benioff and D.B.Weiss way of thinking.
Lyanna Stark has been referred to, in the TV Show along with Rhaegar Targaryen countless times. Even the last season 5, we found Sansa going to the crypt with Littlefinger to visit her aunt Lyanna. I'm sure if you think practically, its quite evident where this is going. The writers won't waste the show's precious time by making a foundation for introducing the best kept secret of George R.R. Martin.
There is another theory which goes by B + A = J. There are many who support it and many who don't. It suggests that Jon is the son of Brandon Stark and Ashara Dayne. There are some who've worked hard to match the timelines too just to make the theory work. But let's think like the show-writers. If we're not planning to introduce the big secret in the coming season 6/7 then why give so much importance to the crypt in the first place. The crypt of Winterfell has been in the foray since the very first episode of Game of Thrones.
The writers could've skipped many dreams and historic references that the characters experience; but they've carefully built up the hype around Lyanna and Rhaegar's relationship. Some refer to it as love and some as rape. Sansa thinks Lyanna was raped but Littlefinger suggests that Rhaegar showed the whole kingdom he loved her over Elia. How the writers plan to reveal it; is something only they know. But we can be rest assured that Jon Snow is not only coming back he's going to come back with the worthy parentage of Lyanna Stark and Rhaegar Targaryen.
Let's just hope Brandon has been working on his greensight; so he can finally reveal "someone's" true parentage for once.