Kassadin needs help

He is currently sitting at a 46.79% win rate, which is compounded by the fact that he is picked in just 1.29% of games. This means that he doesn't have a lot of noobs and casuals naturally pushing down his win rate as they do with more popular champs, and so his lacklustre win rate is largely from being played by his small but dedicated fanbase. In short, for his play rate, his win rate should be much higher.

So what's wrong with him?

Well, his general damage output is feeble compared to how squishy he is, and his poor early game got even worse with the minion aggro changes. He was always meant to be a late game monster, but games no longer go on long enough for him to scale, and his only real way of doing mid/late game damage is by spamming r, which causes him to run out of mana fast, at which point he becomes useless again. In short, he's got a poor early game and a one-dimensional mid/late-game.

It might be a good idea to increase the scaling on his w, because right now, there is not a single person who levels that more than once. Also, his q needs a rework; the magic damage shield he gets, combined with his MR passive, makes him feast against low-tier mages and famine against everyone else.