Amanda doesn't have to be a long arc, as I said. It can be 3 episode max. She doesn't need to be Blake & Alexis daughter. She can be a child Blake had when he was having an extra marital affair, maybe with an english woman he met during a business travel. Imagine how much Alexis would hate him for that, especially if the affair occurred soon after they got married. It can fuel the Alexis/Blake feud. Also it can give deeper doubts to Cristal, about Blake's true nature, and doubts about her own mariage with him.
It would be fun if Amanda was a genuine good person (writers hate that kind of character, hence the 2 or 3 episodes max) wanting to discover her family. Meeting with Blake, Fallon, Adam, and realize how dysfunctional they are, how selfish, scheming and manipulative they are. She finds out that Blake only worries that she will ask for money. That Fallon doesn't give a damn about her (being Fallon, self-centered on her own dramas). That Adam is a creep, kinda hitting on her.
At the end she tells how disgusting they are, that she's returning to London, and that they will never hear from her again. It would be a nice pun to the OG