I'm all for having Cissnei as a secret playable character. Of course, you can't recruit her until after they find out the truth about Zack. While Tifa kept her mouth shut about him, Cissnei would likely reveal everything on the spot if she were in the party early on. She could join as a way to make up for not being able to save him.
Maybe they could show what happens to Tseng after Sephiroth attacks him, like his reaction to finding out Aeris is dead(the Turks never seemed to notice she was gone). It's amazing how for nearly ten years, we were led to believe he was dead due to a mistranslation when Elena confronts Cloud.
...Well anyway, that's my 2 gil.