I've tried countless iterations of Ekko JG and hybrid builds (ex: Runic-Abyssal + IBG but not full tank) are just more effective and fit more compositions too. They let you have enough mana to frequently clear camps and also have some left over to gank. They give you more utility (your laners have damage anyway) and they're A LOT cheaper on a JG budget. Moreover, it's hard to get 40% CDR with an AP build while still buying Void + Rabadon's (especially since Runic Echoes has no CDR and Transcendence is useless for too long when playing a jungler). The resists in a tankier build also help his shield and R heal even if it's true that they are much smaller. Besides being far less risky and not getting one-shot by everything if you get caught and don't have R, you clear the jungle without taking any damage. AP Ekko still gets chunked a bit until 2+ items.
So, what I think would be nice is if he had just a 3-5 more armor early on or even scaling. I'm not sure what can be done though because I realize he's played mid too and if he gets too many tank stats, maybe it would hurt Talon & Zed. I feel like he's really close to being a decent pick in the jungle but STILL not quite there. He's viable but requires twice the effort compared to junglers who are objectively much stronger and built for it, such as Xin, Eve, Rengar, Shaco etc.