I can see where the idea is coming from, but in the books Aegon is alive - thus being the third head of the dragon, but as I said Meera does look Stark born. There's also speculation that Tyrion is a Targaryen bastard; which TBH I find hard to believe.
The one question I actually have left in the show and anthology is - who would be Dany's heir ?
Seeing as she can't have children and I don't see Jon staying alive after the war.
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