Making Kha'Zix mid an option without buffing him in the jungle at all
My idea would be making so Kha'Zix's Q returns CD (or half CD) and mana on MINION kill. This way, he'll be better at farming under tower, pushing waves alone and lanning in general since he'll have his main skill avaiable more often to trade there, meanwhile, his ability to jungle and fight against other junglers is completely untouched.
I don't think this alone can make Kha'Zix a good pick in the midlane, but at least he'll be playable, which I think will be cool.