It’s pretty clear that Gwangjong’s children will be born only after HS’s death. The question which suggests itself is, of course, why he fathers children, why he becomes ruthless, why he consolidates his power basically becoming a feared, absolutist monarch, why he emancipates slaves or why he introduces Gwageo (the national civil service examination with which he expelled from the court people from powerful clans and replace them with civil officials recruited by merit, giving everyone, not only the rich and powerful people, the opportunity to work for the state.), beside the fact that HS inspires him for some of his reforms.
if you think about the fact that So’s main reason and objective to win the throne is to be with Hae Soo and protect her, and the fact that SHE HAS ALREADY BECOME THE CENTER OF HIS UNIVERSE, you have to wonder how he managed to live 20 years without her. The answer is in the vision of him at the end of episode 8, just before the camera zooms in on his face - standing alone on the platform HE LOOKS UTTERLY ALONE, COMPLETELY DESOLATE - in the close-up he looks ruthless but if you look at his face in the long shot there is a devastating sadness and loneliness: BECAUSE HE IS ALREADY LIVING IN A WORLD WITHOUT HAE SOO, IN A WORLD WHERE DEATH TOOK HER AWAY FROM HIM. It actually looks similar to the final scene featuring Yinzhen in BBJX where he is overlooking his empty palace after RX died.
So why does So continue to live? Why does he do all the things mentioned above? What is his reason to live? IT’S BECAUSE HE LIVES TO BUILD AND CREATE THE WOLRD IN WHICH HS/GHJ WILL BE BORN ONE DAY. In the novel/cdrama, RX told the 4th Prince she came from the future and in the end he believed her. I think the same will happen in Moon Lovers - Wang So will believe her BECAUSE HE WILL NEED TO BELIEVE THAT SOMEWHERE, EVEN IF IN DIFFERENT SPACE AND TIME, HIS HAE SOO, THE ONLY WOMAN HE HAS EVER LOVED, IS STILL ALIVE.