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.
That would have altered Doctor's character development as well. Most likely at least. This whole thread is about speculation in the first place. So I speculate.
Perhaps both Seven and The Doc would have remained more on a same level in their social abilities and Kes would have had to step up more as a mentor for them both. I would have rather seen Kes mothering Seven instead of Janeway as well, because the whole "mother goose" thing Janeway had was a turn off for me. Never cared to see that. Of course that would have not been a lot of character growth for Kes, but it could have provided interesting sub-plots, like Kes had with Doctor in an episode "Tattoo".