Break the Wheel

My theory assumes that Daenerys ascends the throne, with or without Jon by her side. She wants to “Break the Wheel” but, as Tyrion pointed out, that could take more than one generation before her plans are fulfilled. And how will she achieve this without an heir, considering that she can't bear children?

By this time, Jon knows that he is indeed a Targaryen. Even though he was from a bloodine known for their madness, being brought up by the Starks turned him into the man that he became.

Shortly after the defeat of Cersei Lannister, Tyrion presents to either Jon, Daenerys or both an infant who has become orphaned during the battle. He may or may not reveal who the birth parents are but will reason that considering how Jon turned out, the child can be raised to “Break the Wheel”, regardless of who its birth parents were. Daenerys and/or Jon agree and adopt the child as their own.

It is later revealed that the infant belonged to Cersei Lannister. To me, that would be a bittersweet ending because even though a Targaryen has ascended to the throne, the Seven Kingdoms will ultimately be ruled by a Lannister without any guarantees that the Wheel will be broken.