There is honestly no reason to pick Kha'Zix.

The champ is fun and he has great burst damage when enemies are isolated, but as it stands his damage is just way too inconsistent. The new masteries don't benefit him whatsoever as none of them appeal to his playstyle. The only on remotely getting close to it is Stormraider's Surge and even then he has too inconsistent damage to make the best use out of it. Item changes hit him a bit hard as well as the new jungle so it's hard to perform like he used to. Most comps now are early game champs that use that strength to outdamage him, pink wards are cheap and now that people are buying them it shuts down his ability to juke and try to outplay opponents.

Other assassins perform their jobs way more smoothly and safely than Kha'Zix does. He needs something to work in this meta. Some suggestions:

-A buff to his mana maybe? I've never had this problem before preseason and now I'm going oom on my clears. I've never had mana problems up until now and I guess it's having to blow more abilities killing the creeps. You have to hog blue buff from your mid laner to consistently clear.

-Isolation should shrink as you level up, remove it from turrets, or shrink as you put points into ult. It's too hard to isolate champs properly. It's almost impossible to find the right moment to attack and try to clean up. You rely on your team doing good and the enemy being stupid to do anything. Otherwise you just sit in the back and watch your team die.

-Have his ult maybe allow you to mark the next target you strike as "isolated" so you can receive the damage from your Q. Kha'Zix is already a fairly unsafe pick. Nothing hurts more than jumping onto someone isolated before a minion or an ally walks up and gets rid of your passive. This either kills you or you end up blowing your ult and flash just to escape. There simply needs to be some better interaction with his kit.