It's too early to guess anything. And I don't trust Hong sisters for happy endings
I have my doubts too about the ending especially after knowing that we are very likely to get a Part 2, I fear an ambiguous, open ending up for wild interpretations. I hate when they do that. Gives me no closure.
Well if you go by the theory that Jang Uk is not Jang Kang's son but is really the King's son, which makes Jang Uk the current King's nephew and hence Crown Prince would be his cousin. I don't agree with that theory because DNA is all biology and not related to soul, but hey this is a fictional world of magic so maybe Jang Uk really should be considered the King's son.
https://www.youtube.com/watch?v=yfzStIaxhmE