If you want to make Kassadin higher risk...

... then shift his damage onto his W so he needs to stick around for a bit. Redo the active on his W so that the next 2 or 3 hits do a significant amount of damage compared to what it does now. Make the buff disappear in a shorter duration than Zhonya's duration. Then remove some damage from his Q. Now he has to stick around for a little bit to do his full damage. For added bonus he gets synergy with his passive.

Back in 2010 I suggested this, which is basically what he has now. With any luck maybe Riot will listen to me again (because I was obviously the sole reason for the first change).