As you can see Kassadin has a very low winrate. I tried to play Kassadin he was very fun to play only some things bothered me for example I was very fed in most in my games but I still couldn't kill an enemy in a full rotation even in lane I couldn't kill anyone at 25% hp without using ignite. Before you say he shouldn't be able to 100-0 a squishy well full ap Malphite can do it, Lissandra can do it, twisted fate can do it Viktor can do it i can name some more but i think my point is clear.
Add ap scaling to his ult.
If you think this will make him OP then make it scale like (0.4 ap ratio at lvl 6) (0.5 at 11) (0.6 at 16)
Makes his cooldown lower after a full rotation he has nothing to use other then his ult.
Increase the ap scaling on his abilities.
Increase base damage.(his base damages are really low)