Since Kassadin has been in the bottom bracket of the performing midlane champion here's a small idea for him:
instead of the ~0.5 second cast time on his ult, make it instantly cast, but make him untargetable for 0.5 seconds. So basically that he disappears from the map for 0.5 seconds and then reappears in the target area. This makes it seem more like he just jumped through the Void to cover the distance and allows experienced Kassadin players to dodge usually undodgeable abilities, kinda like Master Yi can with his Alpha Strike, although he has a longer time of untargetability.
With the recent change to Malzahars Passive I really don't believe that this is broken, since it is only a really small window of untargetability.