Spellblade is only dmg item on Tank Ekko; Tank Ekko is #1 :D

The is what's most responsible for making the tank build so ideal for him. Take on-hit interactions off of his E, and have E only scale with bonus AD and AP. Also take the slow off his W if he doesn't hit it. This is a substantial nerf, but still allows Ekko to keep his identity while also probably still being useful for good players. The spellblade/slow + E + z-drive too reliably strong. If Ekko wants Iceborn to proc, it should be the AA after a spell (including E). Of course something would be changed in his passives to compensate, but that is a lot of numbers and experimentation.

tl;dr Make the hit after the dash on his E count as a spell only with only bonus dmg scaling.