Reduce the range on his Q!
One of Kassadin's problems is that he can land his combo on you and teleport or walk away, while you are silenced and slowed and cannot hope to counter attack. Even though he is on cooldown, he is out of reach by the time you are ready to react. He continues this cheesy strategy until you are low health, then he teleports on you and slays you without taking much risk.
Reducing his range would force him to get near you, exposing him to your own abilities and auto attacks.
I think this change would maintain the cool play style that Kassadin has, while making him a more reasonable opponent to deal with.
To help him maintain some viability in the SR lane, he could be given a short ranged auto attack. His weapon would briefly extend to the target he is attacking, sort of like Jarvan's Q, but with a more slashy animation.