Let's talk about terrain generation mechanics

These champions are fun to play, in no small part because they can bring something to the team very few other can: Mobility Denial. However I'm sure many of you have noticed these (Azir's ult aside) aren't terribly amazing against the many dashes blinks and otherwise the game has at this point. Yes you can time it when their flash is on cooldown and what not, but why don't we brainstorm ways to add more interaction to their respective terrain spells so they can be used in more cases.
e.g. Anivia can re-cast her W for a no-damage 20% slow around the wall's area.
/discuss