If you wanna go that route, why not Saavik? She finally decided to embrace her Romulan heritage. Would have tied-in with Spock being around for the story, and been a shocker for the fans.
The original script for TUC had Saavik in place of Valeris, however Nick Meyer, Leonard Nimoy and Gene Roddenberry agreed that it would be wrong for Saavik to be a traitor, since she was regarded as a fan favourite, not only that but also her motivations would be complicated...for example:
A)
Saavik doesn't want peace between the Federation and the Klingon Empire, because they killed David Marcus, The Grissom Crew and if not for David stepping in, they would have killed her (in TSFS novelisation, it was stated that Saavik and David were a couple), however "Revenge isn't Logical" and thats the problem, Saavik, although half-romulan (see TWOK deleted scene) exhibited a dominant Vulcan personality, therefore it would be a mess for them to attempt to explain away
B)
Saavik is following orders from her superiors, namely Admiral Cartwright the problem with this is, as we've seen Saavik is quick to question command decisions, she also wouldn't betray her mentor, Spock and indeed Captain Kirk (who greatly helped her career) with orders from a superior with questionable motives
there could be other reasons, but those are the two I can think of, as you can see, in TUC it would be hard to justify Saavik's betrayal in the story, so what makes it any different if she were to betray the Federation in "Redemption"?
For starters, given Saavik has been essentially raised in the Federation way of life (and from what we've seen she's happy with it) why would she suddenly decide to help the enemy, sure it would be a mindfuck to the fans, but with every villain, comes a story, a reason for their "descent into madness" and I just can't picture anything working where Saavik is concerned