Zilean is a specialist, he's a unique champ that doesn't quite fit in with the rest because his main gimmick (Time manipulation) allows for a combination of zone control & utility. But what if we took that a step further? What if, let's say...he could take things BEYOND the norm?
Concept is simple. Zilean continuously stores XP even when he's level 18, why not let him use it to propel both himself and his teammates beyond Level 18? Gameplay-wise it's introducing a unique element that's only been seen in URF (going past Level 18) & further solidifies the fact that you shouldn't be letting Zilean get to late game. Thematic-wise it makes sense as well, Zilean is known for manipulating time & to break the conventional limits of the game wouldn't seem too far fetched in his case.
So, what do you guys think? I think it'd be really interesting to see Zilean being capable of this, especially since Levels beyond 18 aren't too impactful on the game (it's additional benefit from scaling base stats, that's pretty much it). I don't think it'd break him outside of making his synergy with Udyr insane.
Edit: [A commenter on Reddit had a genius idea in terms of adding this into the game, thought I'd share it here.](http://prntscr.com/khqenf)