I'd like Kassadin changes as much as the rest of you, but I'm not sure how you'd go about doing that without making his barely average viability on SR become the new-old Evelynn.
At its core, the reason why Kass is so cheesecake is because of his Ult letting him go anywhere with ease, correct? I suppose one option could be to reduce his Ult's range, if only at earlier ranks? Only thing is, his SR early game now becomes difficult even after 6 because his SR roaming potential suddenly falls through the floor.
Another option could be to increase the debuff duration for increased mana costs from consecutive casts. In this case though, I'd completely shy away from the mana ramp up itself, otherwise once again you're mauling SR Kass.
I mean, I don't think nerfing his damage, or even his passive is going to change his cheese. At the end of the day, his insane map control is what's making him powerful - heck, you could probably remove PQWE and he'd still be viable with a piecemeal crit build after Lv6. Hyperbole aside, any meaningful nerf WILL come in the form of nerfing his Ult somehow. It's just a question of how.