[JDP]

I don't see Arya being the head of the dragon either - she's been telling her own story from the very beginning, not going along with everyone else's. I think that's how she'll continue.

Also, she's not heir to Winterfell or the Crown of the North or the Riverlands - Robb willed the Crown to Jon in the event of his death, and Sansa's the heir to Winterfell as the older sibling, and I guess the Riverlands as well (only if something happens to Edmure's child - and assuming they get it back from the Freys).

It's possible that the third head of the dragon could be Arianne Martell, though I find her extremely annoying. The Prince of Dorne had been planning to marry her to Viserys, Dany's late brother.

Alternatively, Bran could be the third head; he can use his Warg abilities to control a dragon and there would never be any question of funny business with Dany (or Jon... eeeww...) because of his physical limitations. This would also balance out the Targaryen/Targaryen-Stark/Stark equation. No Dragon blood in him though, which might make this the wrong tree to be barking up.