How to fix Ekko!

Remove him. YAY! I balanced the game better than Riot!

No - I mostly jest. Instead we need to look at his toolkit and decide what needs trimmed.

Just a cursory examination tells me that his passive, the W passive, and his ult are the problems. His E has problems of it's own, but those are mostly just the cooldown in nature.

So - how do we fix this... He has a passive pseudo-execute built into his autoattacks - no. Just remove that damage entirely. There's nothing that indicates it even exists, it has zero counterplay, and it's baked into a completely different skill. Fizz has a very similar passive effect - however it's not built into a powerful ability like Mr. Veigarstun+RivenShield has, instead it just does slightly more damage.

Next - His passive - a 3 hit combo with an 80% AP scaling and a 140% movement speed differential... that triggers on spell hits and applies spell effects? Literally no one else in the game has an ability that powerful and potent - and the one's that come close can't do it to multiple targets. Lets remove the MS boost he gains, and tone down the AP scaling - lets say 40%. If we feel this is too much of a nerf, allow it to trigger more frequently.

Finally his ult - Why does this even deal damage? Not even why does this deal so much damage, just... this should not do any damage at all. But - lets keep it, sort of. Here's my suggestion - Remove the damage component of his passive completely. Instead when he ults, keep the big ass sphere of doomydoomdoomness - but instead make it trigger 3 stacks of his passive on each target hit and give him a 15% MS boost for each enemy champion that it triggers on. This makes this interesting - because we're not removing the damage component, but we're toning it down (because it's already way too much) - and we're making it so that he can't use it to execute a target that he just bursted the snot out of, because the passive can't trigger, but we can also use this as an actual escape tool - if someone is standing right there waiting for your ult to CC you, you can't get out of it, and if no one is nearby you don't really get much out of it, but it would make for more interesting plays, more decisions for the Ekko player, and it would make him a funner champion to play against.

Also - his ult should only gain half benefit from CDR. The mechanic alone is toxic, having it every 40 seconds is just... not healthy for a game ever.