We really can't use Kassadin as an example of what the Void does to people.

Kassadin's design is old, with likely a much older version of the Void. The old version was "The Void just fucks ya," when ya go in it. Story telling wise, when the game was focused on the Institute of War, it was probably fine to make a place that was immensely deadly to people who go in, cause it wasn't the place that matter, but what came out of it, such as Monsters. Monsters that came from such a deadly place.

Since League is moving towards more conflict between nations, factions, etc, the Void being such an innately deadly place, doesn't work anymore. How can ya truly 'beat' the Void, if ya start to go purple like Kassadin. Even if the Void won't be beaten in Lore, just like how Noxus or Demacia will never truly conquer the other, that doesn't mean you can go balls to the walls and make it impossible anyways. The idea, that it could be beat, is important.

Which is why, unless you want to limit the Void's interactions, as in The Void, inside it, we can't have the Void be as it was with Kassadin. If it stays the way it was from Kassadin's time, that heavily restricts the potential stories. You could never have a Noxus/Demacian army go in, or individual heroes who then just become like Kassadin.

No, if anything should be focused on, it should be the creatures that live inside the void that makes it deadly. Not The Void itself. What someone mentioned was right, the real problem of Kai'sa would be the isolation from other people, not from The Void. Reread Nami's comic, where she comes close to what's possibly a portal to the Void, and a Void creature, and look at Rasho. That third eye. Just like Malzahars. Someone who gave in, and let the Void in.

Meanwhile, Nami, was able to resist it by coming to her senses. Once she broke free of that first hold, it's influence on her waned. She was able to see clearly, and fight off Rasho.

Now, maybe Kassadin was weak for a sec, gave in, and then broke free of it midst-conversion and that's what's responsible for his state. Malzahar sacrificing his daughter to the Void, broke him for long enough for the Void to take hold of him, but not long enough to corrupt him fully before his anger washed over it. Speculation, but would fit Kass better into a less "Just don't go inside" Void.

Anywoo, that's just my late midnight thoughts.