Ekko, OP, Overtuned, or Just not known how to counter yet?
First of for all the Ekko hate and op cryers, i fill like they are people who haven't played him and don't know how to fight him yet, got destroyed by stupidity and then proceed to cry OP!.
From what i have observed and my own experience, (admittedly my personal Ekko experience was two custom games i played to get a feel for his kit) I feel the reason he is a problem is twofold, both overtuned and lack of established counterplay.
I agree that the %missing health damage on his w is a little strong, which they said themselves it might be and would probably be tuned if necessary, but that is what makes him an assassin instead of a utility mage or support. Because keep in mind the enemy has to be below 30% health for it to proc. Once it does, especially with high amounts of AP, he will eat targets, but to use it he has to follow the same patterns as most other assassins, skirt the fight see an opportunity and take it, then escape.
The only other thing that is a potential issue would be the ratio on his ult, because full AP ult hurts with Ekko, but its probably going to be set high until people get more comfortable using him and make his ult more reliable through experience.
Everything else has clear strengths and weaknesses. People just need to pay attention, learn, and adapt to him.
Open to discussion, i would ask to keep it civil and see if we can hash out together methods to combat Ekko so we can spread them around before they kneecap Ekko, but i do not have high hopes to not see trolls or flames.