Personally, I don't think you ever get the betrothal between the two. I think Aegon falls for Arianne when she comes to bring her own marriage offer, marries her instead, and then Dany obliterates both of them, possibly when she torches King's Landing.
So I think this is the most commonly accepted theory and if I had to pick a direction I'd probably pick this one too.
I didn't used to feel this way for 2 reasons: 1) Because I put to much credence into what the show was doing and didn't believe that they'd leave fAegon out if he were to play so an important role in the endgame like this. I no longer believe that relying on the show for guidance is terribly helpful and 2) I would always go back to Maggy's prophecy to Cersei. When she told her about the "younger, more beautiful Queen" my mind immediately raced to Daenerys.
But upon further review, is that George's fault or my own? George has never hinted that there aren't other women in the world more beautiful than Cersei. The mind just relfexively goes to Daenerys because she's one of the primary characters, is vying to be Queen, is young and beautiful, hates the Lannisters, and we, the audience, know she's good and Cersei is evil, in the loosest sense anyway.
But now I believe it's referring to Arianne. Arianne is by all accounts also very beautiful, is vying to be Queen, and hates the Lannisters. But what really puts her over the edge imo is that she actually is in possession of the thing Cersei "holds most dear." Her child. She has Myrcella. And I can envision a lot of ways that she may "take all that Cersei holds dear." Perhaps by simply turning Myrcella against her entirely. Or something far more cruel, like killing her in front of the Gates of King's Landing.
Finally, Daenerys has more prophetic dreams and visions than any other character save Bran. She sees visions and symbols of, in no order: Jon, Rhaegar, Viserys, Tyrion, Euron (or Victarion), the Red Wedding, Aerys, Ellia, Aegon and Rhaella, the Army of the Dead, her unborn son, the burning of King's Landing and that's just to name a few.
But she never sees Cersei. Not once. Not even a reference to Cersei. I think that's telling that Cersei's final confrontation isn't with Daenerys, but with Arianne...........and then Jaime
