Would definitely like to see Iorveth and Saskia back. Would certainly make the choice to pick Iorveth's path more meaningful. Also selected Adda. She was mysteriously absent despite apparently being the queen of Redania now.
Wow, the battle of Kaer Morhen and love triangle Letho - Roche - Iorveth? Roche would have a heart-attack.
I would like to see also some other characters, but most of them might be dead based on players' decisions or/and irrelevant for the Witcher 3 story. So I understand why we haven't seen them.
I would probably prefer Adda (interesting female character, what she thinks about Radovid?), Cynthia (Nilfgaardian spy, makes sense to meet her), Siegfried (interesting character who fights for Redania, but there is witchhunters/Flaming rose problem) and Anais (would be nice to see her in Roche's camp/Radovid's ship).
John Natalis is not bad, but Temeria fights a guerilla war now. Lady of the lake makes no sense. Vincent is happy with Carmen. Abigail is fine, but not important enough.
Saskia has no flaws and is too powerful. This combination does not fit into the witcher setting. And imagine a dragon on your side in Kaer Morhen. Would be nice to have some better info about her though (i.e. from Iorveth).
He might have survived, but the regeneration might take him hundreds of years. And OP suggested only characters from games. Anyway I would like to meet Dorregaray of Wolle and help him to save some endangered species.
I guess it's too much to ask for an expansion in which Borch and Saskia lead a huge army of sabre wielding Zerrikanian amazons invading the Nilfgaardian empire, first crushing that pathetic elflings state in the flower valley... irate: