HERE IS MY CRACKPOT THEORY. I posted it right after reading adwd but it's time for primetime!!!
It's based on the idea that all
of Melisandre's visions are true
We know that Melisandre once saw visions in the flames of Stannis as Azor Ahai, being the one to stand up in the final battle against the The Great Other. That's why she came to him in the first place.
Lately, though, she's seen visions which clearly tell the reader that Jon's Azor Ahai ("I ask to see Azor Ahai, but I see only Snow,").
So, she's seen visions that both Jon and Stannis are Azor Ahai.
Later, she tells Jon that she saw a vision of him as man, then wolf, then man again. I'm thinking this means something that literally will happen (Jon will "be" himself, then warg ghost, then warg a human).
So here's where it gets ridiculous. Jon died at the end of adwd, but warged into ghost (ok not that crackpot yet). His body, however, is totally
unrecoverable what with being riddled with holes, and it's burnt soon after the end of adwd. So, Jon is stuck in Ghost, and to fulfill Melisandre's vision, needs to eventually warg into another human vessel.
Now for the final twist: for some reason*, Jon ends up WARGING INTO STANNIS. That way, it's Jon in Stannis' body who eventually becomes Azor Ahai, and all of Melisandre's visions are proven true! I'ts only TOGETHER that Stannis/Jon can be Azor Ahai and save Westeros.
*So why would Ghost/Jon warg into Stannis? I can think of a few reasons. Maybe Stannis is dying, and Jon warging into him somehow saves him... Or maybe Stannis is about to do something horrible and Jon wants to stop him. Or maybe Jon finds out he needs to do it for some reason to fulfill his destiny.