How to fix Kassadin with one mere change.

Increase the cooldown of his Q at early levels for a ridiculous amount.

Something like 22/18/14/10/6. Just a completely insane early game cooldown.

Why?

Because Kassadin's strenght in lane comes from his pre-6 trades. Thanks to the CD and damage of Q, and the sustain of Flask, he can Q every time it's off cooldown, and once he reaches lvl6 he's at the same health as his opponent, even if they have been harrassing him all day (Talon being the exception). By massively increasing the early game cooldown, kass won't be able to "trade" at all. His opponent will almost always have better health than him. That way he can still maintain his late game monstrosity and allow opponents to shut him down easier during lane. A good Kassadin shouldn't win lane. A good Kassadin should SURVIVE it. This way his ult range, the thing which defines him the most, can remain intact.

If this proves too much, increase the AP scaling of Q or the base damage of later levels (Maybe even add scaling AP ratios. At lvl1 it'd be less than it is now, at lvl3 it'd be the same, at lvl5 it'd be better) or reduce the scaling mana cost of his ult.

If this doesn't work, just increase the range per level of ultimate. 500/600/700 range would probably be enough.

Just don't destroy the Void Walker, please.