PROOF THAT EKKO CAN BUILD ANYTHING AND STILL DO WELL (MINI GUIDE)
I haven't gotten full build with it yet, but the whole build would be:
CORE
OR
SITUATIONAL FINAL ITEM
Tanky
Damage

One of the reasons this actually works is because of Ekko's W passive, which adds ADDITIONAL % missing health damage to opponents below 30% health. So, with guinsoo's you will be doing 20% max health damage every other auto attack (not factoring armor or magic resist), as well as % missing health damage. This only really works while jungling, due to bloodrazor being a jungle item. You could try it in lane, if you feel like it. For that, take skirmisher's sabre, so you at least get some use out of the smite summoner spell.
WARNING: you may not want to try this in ranked at all or at least until you are used to it. It's funny at first having to auto attack a lot on ekko, but once you get used to it, you're basically a master yi with 10 times the utility. I recommend this for having fun in normals, and really only when you get ahead. I have played this twice and gotten an S- twice.
Also i dont know if this is in the correct board section, so please forgive me if it's not :P