@risaruart With each episode, number of theories are increasing, lol
Let's see what happens..

As JulySnow2 had already mentioned before, I also have thought that the king has been reincarnated as Sunny as I watched the 6th episode. In this scene where KS tells Eun Tak about his sins we see the flashback to the queen and the king and as the kings face fades out Sunny's face blends in and we get a long face shot of her as if we see continue seeing the king.
As to why GR can't be the queen bc it takes many sins, maybe it doesn't take many sins to become one, maybe one grave sin is enough. For example to advice someone to do something that leads to deaths of many... (I'm talking about how she was the reason why Kim Shin stepped forward).
Plus we once got the info that ppl doesn't always reincarnate as same gender when Goblin spoke about the 'young king' being one of the male group idols in the tv but GR said it also could be one of the female idols. It wasn't that serious and was briefly mentioned, but nonetheless  there must be something coming up with gender-bender reincarnated ppl. Maybe its only about some side characters or maybe we got this info to comprehend how GR can be the queen and the other way around.

I really hope it to be so, bc it adds another unusual and exciting thing to the drama.

I'm thinking that for Eun Tak to be able to take out the sword is when she actually marries the Goblin? She's technically not the Goblin's bride since they're not married so maybe that's why she wasn't able to take out the sword

I think it's almost confirmed than Grim Reaper & Sunny are the King and Queen (like 95%)

For example, Lee Dong Wook's character is named Wang Yeo. If you study Korean history, back in the Goryeo times the Kings and Princes have the surname Wang ( a more recent citation in pop culture are dramas Shine or Go Crazy and Moon Lovers). Also you'd see Deokhwa doesn't call the grim reaper Wang Yeo (the name listed as Lee Dongwook's character) but 저승사자 which means yeah, grim reaper lol. So I'm almost sure he was the King.

Sunny is surely the Queen as well as she's listed as Kim Sun (seeing Goblin is Kim Shin which means they're siblings). Back in those times, it's normal that higher ranked nobleman (military or scholar) married their offspring to members of the royal family. For a King, especially with this drama's king who was young, it was very strategic to marry a family member of the general of the military (highest ranked military officer) as this puts the military in check. It is not uncommon in those times for a coup to happen and many rebellions back then are led my military generals so the marriage of military and royalty, in a way, 'secures' the power of the King and protects him against the possibility of this happening. 

What's heartbreaking for me was these scenes: remember when the Goblin rose to life to seek revenge on the King, but saw he was already dead? I have a bad feeling the King's 'advisers' (those who planted the idea that Kim Shin was a traitor) had a hand on the King's death. Also, there was a scene where the Goblin sees the paintings of the Queen. This sounds tragic but my hunch is the young King and Queen legit loved each other. Which is why the Queen's death was so painful for the King (typical kingdom vs love conflict) that he ended up doing all those paintings of her. This might also be the reason why the Goblin said he was 'regretful' when he saw the artworks.

Nevermind, I give up, we have CONFIRMED CLICHE re: Reaper, as of ~ep08, sooo.. since I shipped KS/GR, the "sister" acknowledgement just kills it for me, lol. Oh well, another too-predictable story down, likely still many to come. At least I like the actors ;P 

Aaaand! When Goblin wrote their names Grim Reaper experienced pain seemingly at the same spot the queen was hit with the arrow!

Acc to ep 8 the old lady/lady in red is god right?