I was talking to a coworker about the books and show the other day, and we got on the topic of what we think will happen in the end, and he told me about an interesting theory that I hadn't read before:
Griff/fAegon is actually a decoy/ruse, and is acting as a convenient figurehead until the hidden Blackfyre movement meets it's true namesake... Daenerys Blackfyre.
Dany is actually the hidden Blackfyre, and was sent for hiding across the sea with Viserys, who didn't know any better in his youth.
This is interesting to me, because it would give Jon and Dany's potential marriage and offspring legitimacy, whereas as things stand now, they are committing incest and their children would be bastards. Jon would be the true heir to the Targ throne, and Dany wouldn't actually be his aunt.
I don't know if that means something, or if it doesn't matter at all, since it's unlikely that GRRM lets both Dany AND Jon live through the end, regardless. I guess he could kill Dany after she has a kid, which gives Jon a child that is legitimately an heir, but that still seems a bit too rosy.