Change Ezreal's Essence Flux (W) to Damage Minions
As it is, AP Ezreal is a sub-optimal niche build. He can do pretty good damage if he lands all his skill shots, but in lane he's completely outclassed by other mages, because of his poor pushing ability. He has to blow his ult if he wants any kind of wave clear, which hampers his ability to duel, teamfight, and roam. I think Ezreal could become a viable, possibly high tier mid lane mage if Riot made the simple change to allow his W to damage minions and monsters. To balance the ability, you could make it so that his W didn't proc his passive on minions/ monsters (which would be OP for AD Ezreal, who is in a fine spot at the moment).
Ezreal is a fun champ, and his AP version is one that rewards skillful play. If Riot were to make that one small change, one of League's coolest champs would be a very viable, exciting pick for the mid lane.