I love word-wide adventurer and wanderer-esque champions, it always makes me wonder about how they probably know more champions in the League universe than any other. So apart for the Piltovians and Zaunites, who does Ezreal know from across the lands?
I would make one guess, that being: 
Because Jax is also a wanderer and they both seem to have spent a lot of time in Shurima.
I think that Ezreal has met almost as many champions as Ryze. Certainly quite a few in his travels, but there's a lot more Ezreal has to explore.
He's not been to Ionia.
This is probably one of those "you're not allowed to ask" questions.
Its possible that he met Kassadin since:
Kassadin started life as a lowly offcast, walking the harsh sands of the Great Sai alongside merchant caravans to draw predators away from their more valuable goods. He survived many of these treks across the desert, and began to serve less as bait, and more as a guide.
and:
Following his mother’s meticulous field notes, he crossed the Great Sai with merchant caravans heading south.
I mean, Kassadin even is in Ezreal's related champions tab.
and 
, I can imagine them in an 'Uncharted'-esque game based in Shurima.
I think its safe to assume most champions Ez calls by name in his special interactions, he has canonically met (or Riot has plans for him to).
I think as the two poster boys of league named after the creators (sorry buddy 
you aren't as important)
These two would have very good banter and chemistry if they ever shared a common goal
But seriously it seems like there's a color story coming out soon that may or may not involve Ezreal and Nasus (based on title).
