How to reasonably Weaken Tank Ekko while keeping AP Ekko Relevant
I think an interesting change 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 weaken 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.