I love Saavik and would hate for her to have become an evil character.
I personally think that Meyer is bad at directing Vulcans. Nimoy understands his character and species, regardless who is directing. I actually think that Curtis was more than capable of portraying Saavik in Meyer's universe under his direction. He just didn't think the audience could connect with someone whose emotions are subtle subdued and I disagree.Admiral Cartwright going bad doesn't sour previous films in the series. Saavik going bad, on the other hand, would sour TWOK.
Cartwright's a throwaway character; Saavik not so much.
In fact, if Saavik had been the traitor, the plot could have been tweaked to make her less ruthless, so that Kirk and Spock could win her over rather than resorting to dreadful unlawful interrogation methods, thus redeeming her somewhat.
I don't think Saavik would have overshadowed the originals any more than Valeris did and, in fact, her presence in STV could have added a much needed female presence, and given us some details of her traumatic childhood, which could have dovetailed very neatly with STVI.