Some speculation about season 5 and the reason I think they are building up Teryn (false Ciri) as a character
(spoilers for Tower of the Swallows)
Teryn is going to fill the role of Joanna Selborne (Kenna) in the series and be the one to unlock Ciri`s powers.
This is a scene that needs to happen in the show, as it is such an important moment for Ciri, and there had to be a reason why the show built up Teryn as her own character beyond false Ciri in the book.
with Skellen manipulating her and her going along with being a princess, combined with her not going with Yennefer despite being offered safety, despite the fact that she has been discarded by Emhyr and with Yen warning her he will kill her now that he has no need for her, it is very likly she will be going with Skellen`s Gang (which has already been set up for season 5)
Although we haven't seen her use any magical or psychic abilities we know she must be a mage since she went to Aratuza and there really aren't any other characters who can fill that role since they have to be someone who is sympathetic to Ciri, but can't have met her, and someone with magical abilities
Joanna Selborne is kind of a weird character in the books she just shows up, has part of a pov chapter with her being imprisoned, flashback to her being a part of Skellen's gang and then she saves Ciri, we flash forward back to her court case and then she just exits the story. She has no other role in the narrative and it wouldn't make sense to have to introduce a new character when they can use an already established one
And honestly, it would be much more narratively satisfying to have Ciri and false Ciri meeting each other, than it would just be someone who has no connection to Ciri, but a scene where Teryn who at first hates Ciri and wants to get rid of her due to Skellens promise of her being able to be a princess one the real Ciri is gone, all the suffering Teryn endured because of the search for Ciri, reads Ciris mind and realizes she has suffered just as much, and in that moment she unlocks Ciri`s magic that would be a powerful scene that would serve both of their character arcs