As it stands, ekko's ult heals him for 100/150/200 hp + 20% damage taken in past 4 seconds as base healing with the additional (+ 6.6% per 100 AP). If say ekko's ult healed him for only a flat base amount plus an amount scaling with AP, then tank ekko might have some trouble using it.
For example, give ekko's ult a base 100/175/250 heal (+ ~8% to 10% per 100 AP). This should discourages tanky builds because his ult will heal him or such a small amount, but for AP builds 200 hp + 10% per 100 ap is a lot, like some 200 + 70% lost in past 4 seconds with max AP possible.
This isn't exactly the best way to balance ekko, but at least it further discourages tanky builds.
The same concept can be applied to his w passive as well:
Ekko's basic attacks deal 10/15/20/25/30「 (+ 1% missing health per 33 AP) 」 bonus magic damage to targets below 30% of their maximum health.
I will assume most of this has been tested, but as it stands ekko isn't being played as intended anyway so he might as well be changed.
Edit: Hey, also you could increase the lockout duration of his passive to like 12 seconds or so that decreases with ap down to like 4 seconds.