Kha'zix doesn't need an early game buff.

Yes, Kha'zix is weak and exploitable early on. I think that's actually OK.

Kha'zix is based on evolutions. His power should be very weak at first, then continually scale into late game. The reason why he doesn't do that currently as well as he could is because his abilities have absolutely no synergy.

Look at Viktor, another champion based on evolutions.

His abilities are combo-able and actually lead into each other in a meaningful way. For example, his stun and his E, or his stun and his R. With the evolved stun, it drags all the enemies to the center of his field, where he can hit them with the double laser, or hit them all with a full blast ultimate that they have less ease escaping. Each ability has significant use and purpose that adds power to his kit.

Kha'zix, on the other hand, doesn't really have the synergy that evolution should add to his kit. His W slows the enemy, but that only allows him to catch up, which may not really be necessary if he uses his E correctly. His reset is great, but given how situational his Q is with its damage, it's not necessarily very reliable. His evolved R doesn't really give him anything significant.

How can his abilities have the much-needed synergy? I've seen suggestions about giving him the ability to force isolation, or give his invisibility some sort of other use. What I would suggest is slightly different, and has a bit more counterplay.

Evolved Q should stay the same. It's the bread and butter ability. The damage is amazing, but only when you have good positioning and timing. My only thing is that the isolation should be calculated on cast, not on hit. This means no more random flashes during the animation that negate the mass damage you would have done.

Evolved W needs to enhance his damage in some way without actually dealing increased damage. The way you do this is by giving consistency. Every champion hit by evolved W has a decreased isolation radius. Probably by 125 range. This would put the range around 375 units (the range of evolved Q), meaning if there is no ally closer to them than Kha'zix is, he has isolation damage (this makes logical sense to me). It also allows for the counterplay of tighter grouping protecting against his formidable damage. Gives meaningful synergy between W and Q (even without Q evolution).

Evolved E is fine as is. It's incredibly powerful. Probably not one to evolve first.

Evolved R sucks. Instead of just "MOAR INVISIBILITY", why not actually give some sort of stealth hunter utility? When evolved R is activated, it gives Kha'zix predator vision of all enemy champions (and their isolation circles) within (1000? 1200?) range. This synergizes with his kit, because it can help him determine whether jumping in will give him isolation or whether he'll deal no damage. It synergizes with his passive, and his full damage combo of W-E-Q-auto-R-Q-auto. This also allows it to be a meaningful alternative to another evolution.

These evolutions actually force choices to be made. Do you need damage, or do you need consistency? Do you need the stealth mechanic and its vision/objective/assassination utility? Do you need kill resets? Unless you face Rengar and win an event match, you will HAVE to go without one of them, but you will gain something significant from all of them.

TL;DR

The PBE early clear buffs are a step in the wrong direction. Kha'zix should gain significant powerspikes from every evolution. A weak start is a good balancing mechanic for a champ that has the potential to snowball as hard as he does. Give his abilities synergy rather than throw numbers at him and call it good enough.