Third dimension physic
For example since Anivia fly's she should be able to go over Jinx's chompers without getting trapped, and Rek'sia's burrow would get her under them.
5 Comments
Unnecessarily complicated counters
While you are at it:
can oneshot 
who can oneshot 
can't be blinded
can reset the game everytime he ults (kinda like /remake)
Btw 
isn't a him :I
and she can get past the chompers using burrowed E
This would probably be more an annoyance to code than anything else- having to flag all these different characters with a "flying" passive in addition to their current one, then having a condition or modifier on a ton of different Champions' powers designating them as being land-based etc.
But really though it would just seriously affect game balance in a way Riot doesn't want to deal with and would have problems doing so given the current limitations of the game engine.
Anivia can barely fly 1meter high lol