I think it's an interesting idea. I could see Kes helping rehumanize Seven instead of the Doc, perhaps using her psychic abilities to benefit somehow. Obviously, Seven would probably become the Ops officer, which frankly makes more sense than giving her a non-position as they did.
I wonder if they would have actually let Kes age? If so it would have been interesting if she transitioned from a sisterly relationship to Seven, into more of a mentorship, and then finally Kes dies and the first real human moment Seven has is this - she cries. It would also be interesting if Kes turned into a Jean Grey and came back to life via her psychic powers.
That would have been cool.