Concerning Kai'Sa and Character Design
I'll start by putting my opinion out there; she's boring. Conceptually, she is cool in a similar manner to Malzahar and Kassadin. These are humans who experienced contact with The Void and regardless of how brief their connection, were irrevocably changed into something new and frightening. There are other examples of Void corruption in lore, such as our dear scuttle crab, and whole ecosystems transformed by its presence. What separates Kai'Sa from previous examples is that she was immersed in The Void for an extended period of time.
To survive among predators, Kai’Sa became one. Stranded for years with no hope of rescue, Kai’Sa endured through sheer will and a symbiotic second skin adapted from a living Void creature. Now the Daughter of the Void emerges an apex hunter torn between two worlds: the one that birthed her, and the one that made her.
Kai'Sa didn't just embrace The Void, she came to embody it, body and mind. She behaves as a Voidborn does, hunting, killing, and conquering, a paragon of otherworldly virtues. She should barely be recognizable as human and yet she hardly appears changed.
https://lolstatic-a.akamaihd.net/frontpage/apps/prod/kaisa-reveal/en_US/825493e12db0f1a326872f7529ac4e30c197a01a/assets/downloads/wallpapers/kaisa-1920x1080.jpg This is Kai'Sa. I thought it was Ahri cosplaing Witchblade.
This doesn't look like Void corruption to me, especially not protracted contact with an extremely unstable reality. The design is safe, sexy, and somewhat consistent with previous examples of Void-corrupted champions, but it is not interesting.
If someone spends a great deal of time in The Void, I don't expect them to come out looking human, let alone pretty. I would expect them to emerge as a vaguely humanoid abomination, like so: https://i.imgur.com/56uMm.jpg
These designs are monstrous, maybe even disturbing to some, but they are humanoid. They make it plain to any casual observer this thing was human, but not anymore.
The Void is one of League's most straightforward sources of horror. Every Voidborn champion is a walking nightmare, and every Void-corrupted character is a twisted evolution. Kai'Sa does not reflect The Void, and I find that disappointing.