I think Ekko would be a really well-balanced, fun champ to play as and against if his damage or scaling was nerfed heavily across the board. Keep all the mobility, heal, shield, etc. and he could really shine as a scout/poke support. Sort of a hybrid between Janna's scout mobility, Zilean's zone control, and Sona's poke. That seems fair.
On the other hand, we could look at other champs with his assassination potential and prune accordingly. To put him on par with the already-overloaded Riven, you could completely delete his global presence by putting range limits on his ulti along with deleting his heal.
To put him on par with the already too-mobile LeBlanc, you could delete both his heal and shield and make him actually squishy.
To put him on par with the already nigh-untargetable Fizz, you could delete his heal, shield, speed, and global presence.
To put him on par with Ahri, you could delete his global presence, his heal, his slow, his aoe size, his untargetability, and make everything a skill shot.
The point isn't that he's the best at everything, just that he's too good at too many things compared to far too many champs. His fundamental flaw is that in a mobility meta immobile champs are already struggling. Ekko has the mobility to play ball in the mobility meta while his skills completely hard-counter nearly anything that's moderately immobile.
Beyond that, his constant dashing and blinking obliterates the skill/reward balance of skillshots across a range of champs. Landing hits against Ekko is extremely difficult but he's less likely to be punished compared to similar squishy assassins hit by the same ability. A good example of this is a recent game where I laned against him on Cassio. Even if I hit him with a poison, he'd start jumping around so I'd get maybe 1 or 2 Es off before the poison wore off. If I was anywhere near him, he could dive me, catch me in his massive slow zone, and there was no way out of his ulti. Similar problem on Malzahar where he can jump out of W zone, shave time off the E timer, and break your ulti.
Right now Ekko's only real counters are a hard cc chain, GTFO, or remaining invulnerable. In other words, his counters are a bunch of toxic champs on the ban list, which is not good for the game.