@Riot, Ezreal W rework idea to consider

Ezreal fires a wave of energy in a line, dealing magic damage to enemy champions struck, and mark them for X seconds. Basic attacks and spells detonate the mark, dealing bonus magic damage. (Mark damage is FLAT + .Y AP ratio)

I could write an entire essay on this, and almost did. But instead I'm going to trust Riot's game designers enough to at least understand the basics of why the idea works. It's not a 100% flawless solution, and I could talk at length about ways to fix the small problems, but again I'll just put faith in Riot's team. Riot, if you are interested in me expanding my thoughts I very much will.