Looking at three storylines, I think the character arcs would have been:

Yu Xiuming - redemption arc
Yezi - growth arc
Yi Wuyou- corruption arc.

Yezi’s I’ve already explained in a separate thread and I think YXM doesn’t need much explanation other than he lets go of his obsession with invincibility/immortality for the reward of love and earns a relationship that supports him during his inevitable loss of power. In short, YXM finds his redemption in Yezi and Yezi starts her process of growth because of YXM. I also have to say that while the theme of this story is obsession, I’d also argue that it’s about love and how drives people. Minister Jie and his sacrifice for his son. Minister Fan and his love for his sons and hopes for the future. Cheng Wu and Yun Wen and their friendship with Yezi. Of course there’s the main trio but this is seen strongest in the 2nd lead couple even tho almost nobody wants to see it.?

I’ll focus the rest of this post on Yi Wuyou since the poor guy got shafted by the show.

So YWY is definitely a corruption arc and by corruption, I don’t mean in the bad character sense but in the literary sense that his belief in his story’s lie(that he can be the emperor of Da Chao and thus be with Yezi) and the doubling down on that lie once challenged leads him to a “darker” version of himself/ a tragic end.
Assuming YWY gets more scenes in the beginning, the audience probably would have been given more context on why he likes Yezi. This would have given us a better explanation for one part of his obsession and set him up for his arc. We get a glimpse of this and its result foreshadowed in him coming for Yezi while she and A’Rui are being escorted to Tianqi. This is also possibly when his revenge pact with Yezi comes in or it occurs in the middle arc.
In the middle arc, things get more interesting. He and Yezi meet again in Tianqi but her arc is running opposite of his. Revenge is becoming less important to her. She’s letting go of her obsession but this process makes YWY double down on his as her growth makes them drift apart. Challenged by this, instead also moving on YWY believes more in the “lie” that he should be the emperor of Da Chao. This leads him into the final arc.
Since part of his lie revolves around Yezi, it makes sense that there’s a correlation in their relationship. The more she “grows”, the more YWY “burrows” into the lie and it puts them at odds until Yezi makes the final break. YWY already has his plans in place but this break down is part of what makes his resolve rock hard. Without spoiling things, he completely leans into the “lie” and makes his big move.
His storyline is also touching, complex, and very tragic. YWY is not bad character but it’s just that his story puts him at odds with YXM and Yezi. In fact from the point of view of being an heir of a subjugated kingdom, his feelings are completely valid. And to some extent making a pact with Yezi but then watching her drift into the arms of the “enemy”, it easy to see that it would hurt. But in a story that is about letting go of obsession, his refusal to do so leads to his destruction. Especially regarding his obsession with Yezi. YXM alludes to this in their big fight.
YWY thought they could have a life together but once she showed signs of rejection, it was time to let go. It’s just a shame that YWY couldn’t.
(And for those who feel that Yezi should have fallen for YWY remember that her most important person in the beginning of the story is A’Rui. A’Rui is in love with YWY. When YWY offers to take Yezi away for the first time, this forms part of the reason for her rejection. And while she’s in the obsession stage, YWY will always be off-limits because in her head he belongs to A’Rui. And when she lets go of her obsession, part of the reason is for YXM and he’s already in her heart leaving little chance for YWY to make space romantically.)