Actually a lot of the elements about Celes' character were transferred over (and also partially dropped) in VII. The whole sanity /experiment thing is not a huge stretch from what happened to Cloud though it was another factor that led to his sanity being snapped. Yet in the early stages of VII's development, the SOLDIER program was suppose to carry the whole "leads to insanity" sub-plot which was suppose to explain why Sephiroth snapped but then they changed what Jenova was suppose to be.

Celes was actually meant to be a spy who eventually changed her mind as she traveled with the party but it was removed and later used for Cait Sith in VII.