Someone suggested this for Kha'Zix, and I think that this is a really good idea
I've saw someone posting this on a thread about Kha'Zix, but I don't remember who posted this.
Kha'Zix is supposed to be a creature that consumes and "adapt" to the situation. However, with the current way that they work, Kha'Zix doesn't adapt; there will always be the weakest evolution. For example, Void Spike used to be the worst ability to evolve when Void Assault had damage reduction. However, due to nerfs to Void Assault, this situation has been reversed, and Void Assault has the worst evolution (which is a shame because it's really cool to see Kha'Zix changing colors). Void Spike isn't the best ability to evolve, but at least it is better than evolving Void Assault due to the fact that all you get when you evolve Void Assault is an extra invisibility charge. So unless you are really good at landing your Void Spikes and you don't need to teamfight, evolving Void Assault is a bad idea.
What if Kha'Zix's abilities were reworked to be able to be evolved twice? Kha'Zix would still have only 3 evolution charges as always. The second evolutions would have much more powerful effects, but because Kha'Zix would be limited to only evolve his abilities a maximum of three times, he would actually have to adapt to every situation, instead of just choosing to evolve the strongest abilities. For example, by evolving his Q twice, he would be an extremely powerful assassin. However, he would have only one remaining evolution charge. So he would have to choose wisely. An assassin-focused Kha'Zix would probably have to choose evolving Leap. Other second evolutions would be more focused on utility or tankyness instead of raw damage.
Here is one of my ideas:
Q - Taste Their Fear: Slashes the enemy. Deals increased damage against isolated targets.
First evolution: Kha'Zix gains 50 extra range on Taste Their Fear and his basic attacks. He also deals extra damage against isolated targets.
Second evolution: Kha'Zix's Taste Their Fear now can deal a percentage of the enemy's missing health as bonus damage. In addition, Void Assault gains an additional effect. The first basic attack against an enemy champion after activating Void Assault will make him count as isolated for 5 seconds, even if he has nearby allies. This can only be activated during the first cast of Void Assault.
The second evolution will make Kha'Zix a great assassin during late game, as he can now actually burst down a selected target, and isolation is less of a limitation to him. He can choose to evolve this ability at level 11 again to gain this effect during the mid game, making him potentially a mid game monster, but then he will have to sacrifice the resets on Leap, making him a lot more vulnerable. In addition, by evolving Q twice, Kha'Zix will have to give up the utility from Void Spike. Lastly, Kha'Zix will have to think twice before casting Void Assault outside of teamfights, as he can waste this secondary effect entirely.
Another idea that I had: When Void Spike is evolved twice, if Kha'Zix is within the detonation radius, he will stun the enemy for the same duration, instead of slowing it.
This can be potentially broken, though, so feel free to add your own ideas.
