I'm trying to ignore current pairings and see this without outside details. Other than look similarities and "BABIES," I'm not quite sure of what all they have in common. High intelligence, for one, but I believe Ezreal is probably a bit smarter, and he's not exactly one to keep quite about how smart he is (which could get irritating). I feel Ez's arrogance makes him dislike authority and prefers suggestions, which Lux could use to manipulate perhaps. Lux has a love/hate relationship, as she was forced into the military but forced to like it. Given that, their dynamic would probably be strained at times. Ez has absolutely no political obligations while Lux has to dance through multiple circles, so he probably wouldn't understand her deepest plights. Ezreal, at the moment, has a strange character development where all he cares about exploring. Interesting, but it's hard to get an idea for a champ this way. Lux has obligations while Ez runs about in caves with a flashlight like a child. Both are adept at magic, but I don't have a feel for Ezreal much to be able to compare. It could be the equivalent of "Oh, Morgana and Kayle can use magic, so they could be friends!" (different ballparks) for all I know.
If you wrote completely OOC for the two of them and made it the typical "omg, you're cute, but I'm shy, and you're shy, and we'll awkwardly do things around each other and blush" thing, I guess you could make it happen. But I'm am more than happy to be convinced otherwise if you're wanting to take a shot at it. I like being surprised and having people change my mind :3
(damn, sorry this sounded so pessimistic XD)