She Dies in Childbirth
Now, let's take this to the other side of the spectrum. Say Daenerys does fall in love with Jon, which they seem to be on their way toward in the season premiere. Aside from the fact that they don't know they're related, the two seem to find a sense of happiness in one another (when everyone isn't stressing Jon out about his choices). Though Daenerys believes she can't have children because of the curse from Mirri Maz Duur, a theory suggests that the death of her child (Viserion) paid the way for the life of her unborn child with Jon.
So, voila, Daenerys's womb quickens and there's a baby Targaryen on the way! But, things always happen in cycles, and, much like her mother before her and Jon's mother, Daenerys could pay the ultimate price for her happiness and die in childbirth.