I think the main goal for kha'zix is to evolve what seems necessary in the match, in a sense the summoner is playing god by choosing what to evolve. What I think would be a good way to give him this feel is to give him more than one type of evolution, maybe give him 2 or 3 options per ability. For example have his live Q and pbe Q as options but letting him only have one, in a sense its are they all squishy champs or does kha'zix need to bust up some tanks. Allow kha'zix not to simply evolve whats best but to choose a more in-depth adaptation for the match.
some examples for evolutions:
Q: live or pbe
(fighting squishies or tanks)
W: Live or new evolve where it shoots a single missle but isolated procs passive
(wave-clear/teamfight or single target)
E: live or maybe no resets but stacks, no increase on range, basically 1 big jump with resets or 2-3 small jumps
(flashy clean up crew or more stickiness/ mobility in teamfights)
R: live or pbe
(more time invis in team fights or map mobility)
These are just Ideas I thought of, but I think it would be neat to give him more evolutions than just the 4 usual ones. This could also give kha'zix more versatility than just the which order is the best.