Kassadin is literally only oppressive because of his Q
Just nerf his fucking Q ratio down to like .3 and give him back his ult range and he'll not be oppressive.
Ult range felt good to use, and as a Kassadin main, I'd play him if he did zero damage with his spells, but this ult range is just stupid and terrible to play with.
You could pick Lux against him if his constant burst rotations were lower.