This was the first drama that really affected me whilst watching and after watching it.  My emotions were all over the place.  I didn't know the history, hadn't read any reviews or discussion sites before watching so went into this epic story clueless and with no expectations of the story or characters.  After finishing the drama I obviously read about the history and then I had a clearer understanding of the story.

I have summarised what happened with the male leads and reasons as to why I think there were was shift in their roles, especially considering the history of this story.

At the end, it was the emperor who ended up with the female lead.  I don't know if the reason behind this was because there were conflicts with who should be the lead, or the writers just changed their mind after a few episodes, or the story naturally developed that way, or it was always the writers intention following the real life story of empress ki.  If the reason is because they wanted to follow the facts, then I think it's the correct choice the writers made because when you have a historical drama based on a real person's story and also have another fictional story thrown into this same story then it just doesn't hold any substance as viewers will know the truth and irrelevant plotlines added to the story (Maha's storyline being case in point!) becomes unnecessary.  That particular storyline was a distraction and a tragedy and if I'm honest quite useless.  The purpose of him was purely to create more angest between the main leads and keep the love triangle alive. It certainly didn't bring Wang yoo and Nyang back together so what really was the point of it.

To begin with, either you follow the fictious character's love story (king of Goreo) or you follow the real life character's love story (emperor).  Mixing the two is conflicting and confusing for the viewers.  If one actor (who is supposed to be the main lead) gets sidelined for the other actor to take his place (with more screen time, more importance in the story and amazing chemistry with the female lead) then that tells you where the writers heads are at.  

I think there was enough material to work with if they had just focused on empress ki and her story in the palace.  There was plenty of drama going on in the palace with the backstabbing, betrayals and manipulation.  Identity crisis was also a big theme running throughout the drama particularly for Nyang and I think the story should have focused more with her identity conflicts rather than the love triangle conflict.  

Once Nyang had chosen to be with Tawhan then it was obvious she would stay with him as she's loyal and as she is the female lead she would never betray him or would commit adultery.  The ending also proved her decision to choose Tawhan was correct because of her love confession to him, which she never said to Wang yoo.

Usually in kdramas the second lead never ends up with the female lead, so I assumed in this love triangle the emperor would have a one sided love for Nyang or perhaps remain freinds or enemies.  However, the story quite early on started to focus on the emperor and centered on him and Nyang's relationship whilst the kings presence began to diminish drastically.  I noticed there was a role reversal between the male leads once the storyline shifted to the palace and China.  I've never seen this happen before in kdramas where leads change and I've watched plenty, especially dramas involving love triangles.  

I'd be more than  happy if someone can shed more light into this or say if it's happened in dramas before.

If the king was originally cast as the main lead and supposedly endgame then I can understand the viewers disappoinment if they preferred or had a biased towards the king and watched this expecting he would end up with Nyang because that's how the story was set up at the start.  

My thoughts on why there was a switch in the male leads could be because the emperor's character connected with viewers and so the emperor gained more popularity or JCW's acting outshone the original male lead's acting.  Tbh, there could be many reasons but the emperor was the reason for Nyang becoming the empress, (however flawed he might be) and without him there is no Empress Ki - the title of the drama!

It would be interesting to know if anyone else has the same or different opinions, thoughts, views or comments.

I agree 👍 you could see Nyang softening towards him quite early in the drama and he never gave up on her.

Agree again.  It was pointless, it was also irritating as the story didn't help anyone.  One thing I also never understood was why she would think about Tawhan after Wang Yoo proposed to her.  That means she had already forgiven him for her father's death and had begun to develop feelings for Tawhan.