I think the problem with a Game of Thrones character being a guest is the lack of weapon variety. The majority of them just use standard longswords which could wind up in them just being a watered down Geralt. And while Arya has a rapier rather than a longsword, that brings some of its own issues. Like if both Raph and Amy are in, then she'll be far too similar to the both of them, or if she's in instead of one or both of them, that could cause some significant outrage.
Design-wise, a Game of Thrones character would be a solid addition, but as far as their fighting style goes, I feel like whoever it is would leave a lot to be desired.