How to keep AP Ekko alive while Weakening Tank Ekko

I think an interesting nerf could be to increase the on-target cooldown of his Passive from the current 3 seconds, to maybe 3.5 or even 4 seconds.

Why this will work: -Tank Ekko relies off of the stickiness provided by the Passive and Q (and Iceborn) to stay on Enemies. Over a long period of time (and multiple procs of the Passive), he eventually gets the kill. -AP Ekko relies off of that same stickiness but is able to potentially kill his target incredibly quickly, decreasing the need for further procs (since they're dead).

Result: Increasing the on-target cooldown will nerf Tank Ekko since he requires many Passive procs, but will keep AP Ekko relatively the same since he doesn't need more than one or two.

-From an Ekko main with ~200k Mastery Points on him.