I originally posted these thoughts on a Reddit thread (Discussion:Kha'Zix) someone else started that unfortunately did not get as many views/posts as I think it deserved. (Basically World Championship threads have taken over /r/leagueoflegends for the moment.)
I have been discussing Kha'Zix with my brother for the past week or 2 and we both feel like Kha'Zix needs some very slight buffs. (Edit: these are mostly from my point of view)
First, his ultimate doesn't feel much like an ultimate ability. In order to make it feel more like an ultimate we have discussed that it should receive one or both of the following buffs:
*Invulnerable while stealth. (it is only 1 second of stealth afterall)
Explanation: The damage reduction Kha'Zix receives from his ultimate does not help him in team fights, once he jumps into a team fight he becomes a target and drops very quickly even once his ultimate is used. This alone makes Kha'Zix NOT a viable pick for the late game as he is no help. Even when making him very tanky, he drops very quickly.
*Increased movement speed for a longer duration than the stealth lasts (I personally propose 3 seconds).
Next, his W is underwhelming. The damage is probably fine especially in late game, but the amount healed seems too low to be viable. It makes me question why even have healing attached to the ability except to JUST allow Kha'Zix to be able to jungle without dying before getting to the Lizard camp (assuming he starts @ the blue buff golem first).
For reference, this is the amount of healing done by Kha'Zix's Void Spike: Heal: 40 / 70 / 100 / 130 / 160 (+ 50% AP)
I would also like to note that I think it is completely ridiculous that this ability is increased by AP, this needs to be changed to AD (maybe a much smaller %) or removed and then the heal should be rescaled. No one goes AP Kha'Zix, there is no point to it. If you get TF, that is a whole whopping +15 health restored.
Possible changes to W:
*Change the ability so that it will heal for EVERY target that is hit by the ability.
Explanation: Instead of Kha'Zix getting a flat 40 health restored at level 1 Void Spike when 3 monsters/minions are hit and in range, Kha'Zix would be getting 120 total health restored, obviously this would continue to go up with more nearby targets. A possible proximity change may also help.
*Remove the heal ability completely and buff the damage output of the ability so that it becomes a viable poke.
For Void Spike the way it currently is, it just isn't really very good until late game after you have level 5, it is evolved, and you have a decent amount of AD items. At that point, it becomes an OK ability to clear a minion wave and increases Kha'Zix's burst damage by a small amount.
Unfortunately, as I said above, Kha'Zix really isn't a very viable champion for the late game. He is really only meant for 2 roles: Jungling & Top (though some people have successfully played him mid). And unfortunately Kha'Zix doesn't really fit into those roles in the current meta. He is not a bruiser and he lacks certain abilities that other junglers have. I will say that he is a great initiator and is impressive in the early game, but late game he just becomes useless unless your team is far ahead of the enemy team.