She Dies in Battle
This is probably the most likely end for Daenerys. The Dragon Queen has made her mark as a conqueror, and dying in battle would be an honorable end for her, especially since she's temporarily casting aside her campaign for the Iron Throne to fight the Night King in the North. If she were to die in battle without ever sitting on the throne, this would fulfill another vision Daenerys had in the House of the Undying, where she is called away from the Iron Throne by the sound of her dragons. The three of them lead her across the Wall to a tent where Daenerys finds Drogo and their unborn child, Rhaego, waiting for her.
It's pretty clear what this vision is saying: her mission in the North will take her away from being queen and bring her to the North, where she will meet her end and join her family in the afterlife. And for Daenerys, that might be the happiest ending. She will never love anyone like she loved Drogo and their baby, not even Jon Snow. She's spent years fighting since she lost them, and being back with them, with no concerns of battles or securing her crown, could be her peace.