Why Is Nobody Building AP Hybrid Kai'sa?
Max her Q, then her E, then her W. Evolve her Q. Build nashor's, lich bane, and finish zhonyas after that. It hits the attack speed cap and its damage in conjunction with Electrocute or Comet is fantastic. Don't forget, Press the Attack also works with abilities too, so it's also a viable keystone AP Kai'sa (given that it's still an auto attacked focused build, so overheal can be nice). Get Abyssal Mask if versus heavy AP or GA (last item?) if versus assassins.
Rageblade and gunblade are both pretty bad on her in my opinion.