If it has a cast time, then we might as well keep Flamestrike and give it a knockback component, though tbh that actually does sound cool if it did more damage than a puppy dog's piss puddle. Let's keep it instant, please.
However, the idea of it (BW) replacing something on the L45 tier is actually quite a possibility. All 3 of the L45 tier could become abilities that are either defensive or offensive in nature that also have a level of CC to them. You could, say, put Blast Wave and Mana Shield on the same tier because you either get an AoE knockback/daze (or when glyphed a targeted daze) that also does damage, or a shield that eats a small chunk of mana to negate a decent chunk of health damage, while also knocking back all targets around you when it pops (assuming we were going with the Cataclysm design of Mana Shield).
While I don't exactly have a third talent to add, it would be interesting to see how a damaging AoE could fair with a defensive shield on the same talent tier, because there are many situations where I could see Mana Shield being more viable than Blast Wave, and vice versa, in both PvP and PvE situations, especially if Blast Wave is an instant, 20s cd, with a fair bit of damage tied to it. If a situation calls for requiring more AoE damage or a knockback specifically, I might opt for Blast Wave, while on the same token, if a situation requires a bit more defense, I might opt for Mana Shield (and who knows what third option they could put here!). Both talents being very helpful in certain situations, they wouldn't be overly powerful in any, allowing players to pick and choose what they would like. In short, by placing same-category talents on the same tier (such as L90 talents or the defensive cooldown tiers), people always pick the best one because they all accomplish the same thing, just slightly differently, while Blast Wave and Mana Shield share a small component like knockback, but players would be given a choice to pick slightly more offensively or slightly more defensively, similar to picking the blue or red ring in Legend of Zelda (if they were ~1.25x instead of 2x the benefit).
... Man, I just sorta had a mini-theory there and rolled with it. Enjoy the wall of text about me auto-piloting my brain about the design of how talents SHOULD be.
TL;DR: Blizzard chose talent tiers to share a similar design, and in doing so, they actually created cookie-cutter builds. Placing 3 talents with differing designs offers REAL choice to the players, such as having Blast Wave and Mana Shield on the same tier.