She may not become queen, but I think she's probably pregnant with Gendry's baby (or she could become pregnant from Gendry in the future). If Dany is indeed infertile, and she happens to sit on the Iron Throne (with or without Jon), they will need an heir eventually. I don't think Jon would be the kind of person to impregnate someone else to continue his lineage... and what better way to honor his beloved sister? (I think Sansa couldn't care less about the Seven Kingdoms and would be Queen in the North.) It would also mean that the houses of Stark and Baratheon would finally be united to rule, just as Ned and Robert ultimately intended. Gendry is the only one left with Baratheon blood. If this isn't going to happen it would definitely be a missed opportunity!