I played alot of games with kassadin after his rework. I never experienced his full strenght before the rework, but he was probably extremly strong, because I keep dominating with him late game. Though, he gets easily harrassed, and against a ryze he's pretty much weak. I really wish that they will consider to rework his passive and remove the "15% reduced magic damage" bonus, because it's quite useless with the new burst mages and AD assasins. I think his riftwalk is for now balanced, but it does extreme low damage. as well, it was considered him to be a "mana tank" like ryze, but building mana for his ult that doesn't even scale that well? it helps him to have many rift walks up, but it won't help him to deal more damage.
Does riot want him to be a mobile bruiser? or something different? Build as pure ap assasin makes him extremly vurnable because he can't safely escape while his riftwalk is on cooldown. Zed has his shadows, Leblanc has her distortion, Tristana has her W (since it resets for champion kills), Fizz has his E that gives him invincible for some seconds..
Kassadin instead has to assasinate a target and either survives it or not.
probably rushing a zhonyas wouldn't be bad, but.. how about that his ult restores 50% of his max mana for each champion kill? that would give him at least a small chance to safely escape after assasinating an target.
it's a dumb idea probably, but it might help him.
Sincere