Suggestion for Kassadin's Riftwalk: Make it cost a % of his max mana, not a flat amount

One major problem I see with Kassadin is that the mana cost of his ultimate truncates him into only building RoA, AA Staff, and other high-mana items, otherwise he can't use it effectively later on when the cost is over 1000 mana at full stacks. This means building a Deathcap on him tends to be suicidal, and gives him a sort of "Ryze syndrome" in terms of builds.

I have one idea on how to smooth out his costs so that his builds are more flexible.

(NUMBERS ARE THEORETICAL)

• Change Riftwalk's mana cost to (10 + 5*stacks)% of his maximum mana.

For instance, at 4 stacks, his ultimate would cost 30% of his maximum mana. If he has 2k max mana, that comes out to 600 mana cost.

This might make you think "Why would we build mana if this was the case? The costs would just be higher!" This change would NOT affect the mana-to-damage ratio of Riftwalk, meaning if you don't build high mana, your damage is weaker.

What would this accomplish? It'd make AP-heavy builds far less punishing on Kassadin and more viable.

Would this change work, or make him toxic again?