The Real Issue with Kai'Sa
Kai'Sa is an interesting champion. She's pretty fun, but she's commonly called broken. I don't think she is, too much. She has 2 main issues, though.
First: Assassination. It's all too easy for Kai'Sa to get onto the enemy ADC and blow them up in a teamfight. You can defend against this, though, by keeping all 5 players positioned so that her Q will scatter to them and having at least one player keep their CC up and just blow her up when she ults. Issue is, this is really hard to organize in solo queue. So, the real fix to this is probably actually going to come in with the ADC changes next patch. If an ADC is less impactful in the game, less important of a role, her blowing the enemy ADC up (and dying for it) won't be too much of an issue.
Second issue, though, is actually killing her. She builds Death's Dance and can build Hextech Gunblade, and both of these items come with healing for 15%, 1 from her physical damage only and the other from all of her damage. Combine this with Ravenous Hunter, healing her by 15% for each of her spell damage, and any time she presses Q, she heals for 45% of the damage it dealt. "But Brutalitops," I hear you asking, "it's a multi-target ability and the items heal for 1/3 of the healing if it's a multi-target or DOT." No. It's coded as each shot being single target, so she gets the full 45% from each Q, which is insane. This is an easy enough fix; change the code so that it's set as DoT instead of single target. This will cut her incoming healing from the items from 30% to 10%. The 15% from Ravenous will still be unaffected, but it's much easier to deal with someone healing for 25% of the damage they deal than it is someone who's healing for 45%. One could also argue that Fleet Footwork is an issue regarding the healing as well, but it's getting nerfed next patch, so I'm not gonna make that argument.
oops