I believe that the main couple casting will either make or brake the drama. 

So the best FL & ML in my opinion are:

Shen Miao - Zhao LiYing (here’s a fanart of her as Shen Miao https://pin.it/3Mpbrml )

Xie Jing Xing - Xu Kai or Wang YiBo (we’ve seen them in enough period dramas to know that they really suit this character)

I think Xu Kai and Peng Xiao Ran would be the perfect main leads based on their acting and appearance perfectly fit their description. 

 Snaxx:

My Dream Cast

Shen  Miao - Zhao Lusi 

Xie Jing Xing - Yang Yang / Wang Yibo 

I don't care about other characters 

I cannot even imagine Lusi in this role. Then again I have never seen her playing a grey or villainous character. ¯\_(ツ)_/¯

 Sinasina:

I cannot even imagine Lusi in this role. Then again I have never seen her playing a grey or villainous character. ¯\_(ツ)_/¯ 

She once played a very annoying villain in Untouchable Lovers, the usual rival in love obsessed with the male lead who see herself as a noble princess (come to think of it, I wasn’t able to like Zhao Luis after that drama but I can’t decide if it’s because of her annoying  character or her flat acting)

And after that, she kinda always played the same kind of character, the cute, cheerful girl, totally different from Shen Miao so o understand you can’t see her in that part

 ecwc:

I don't know why I didn't think of it sooner but Xiao Zhan could also pull off playing the Purple Clad Youth, don't you think?

Lol, your comment makes me laugh because literally EVERYONE thought of him for that part

 Vico SDL:

By the way, I’m kinda disappointed because now, we still have no real info  (only baseless rumors) about the production : no director, no producer and the production company is totally unknown so we have absolutely no idea about when it could happen or with who.

I hope this is not gonna be like the movie adaptation of Wicked : they tell us they’re gonna do it but 10 years later, there’s still nothing 

The tightening censorship makes this one happening increasingly unlikely. 

I have not given this much thought before, but considering how reincarnation is a taboo now, it would be pretty difficult to bring this one to the small screen. (they could give it the same treatment what One and Only & Forever and Ever have gotten, but I'm skeptical)

 Vico SDL:

Yeah but I thought they would do like Princess Weiyoung, Jun Jiu Ling, Miss Truth, Legend of Hua Buqi, Untouchable Lovers, and many others : change the rebirth/transmigration trope to make it something different, like a secret identity 

I don't know, without foreknowledge the story doesn't make any sense. To make it work with some significantly different trope would mean that only the character names are the same, it would no longer be an adaptation.