So, I'm of the persuasion that the reason a true sniper champ doesn't exist is because true sniping is boring.
I remember a hero from a WC3 map called Advent of the Zenith that was a sniper, and he did so by an ability which slowed his movement and attack speed, but gave him massive range and a damage buff. It was cool, but as the game progressed he slid into the role of sitting by an enemy tower and shooting it, then blowing all his CC to run when an enemy showed up.
See, it can be made to work, but the reason it's bad is because it's good. It has such insane utility that a sniper's range and sniping abilities have to be hid behind significant retraints (Channelling, CD, Ultimates) so that they are not simply used all the time as opposed to the more fun style of play which is more active, mobile, and meant to interact with enemy players.
Did you ever play Metal Gear Solid 3? It had a boss named The End, and it was a true sniper battle in its first design. You set up your sniper nest, and he does his, and you slowly scan around to find him, then take your shot. Ultimately the fight had to be dumbed down to the level that the game finally had (where the best way to do it is run around til you trip over him, then fire wildly at the old man) because real sniping is... slow.
My point is that, sadly, you don't want a sniper champion. You want the ideas from one, and that works, but you don't really really want one. Cait is a nice start, and there's more that can be done (Don't let me stifle the conversation), but you wouldn't want a full sniper champion. My two cents at least.