100% there's the reason, they keep saying: "The dragons are the only children I'll ever have" and the rule of succession.
Dany is going to realize that she doesn't want the throne because she's not a good ruler. Mereen, and Astapor are prime example.
It's her who will bend the knee to Jon and allow him to be the king of Westeros - thus the Song of Ice and Fire. The tale of Jon, who lived as a bastard to become the legitimate ruler of Westeros.
I don't think it'll go that far (personally I think Jon dies the tragic hero's death at the end of it all), but it could well be possible.