I suspect that if riot were to nerf his passive, reducing the slow and movespeed buff, then he wouldn't be as slippery. The problem with that is that is would hurt squishy builds more than tanky because squishy needs the slipperyness to not die.
That's the problem that Riot's playing with. How to ruin tank ekko without breaking AP ekko for good or bad.
Honestly, I think a nerf to his utility is the only way. Tank ekko has enough CC to be an initiation tank for his team. When you can do that, you don't need damage. If they nerf his ability's CC, that'll force him out of tankko because he doesn't do enough of anything to work anymore.
Having a 2.25 second stun at 1 point, making it such an amazing one point wonder, is I think a good place to start. Assuming Riot wants a stun of that duration, let's reduce it and make it scale with skill level.
1.85/1.95/2.05/2.15/2.25 second stun would be a decent hit without being overwhelming.
This level of nerf would not destroy his utility and AP ekko can still get his 3 hits passive proc during this time, but it's a huge difference for a tank that needs a while of free time to kill a target.