Often the debate on Crowd Control sounds like it is a choice between AoE or Crowd Control. If tanks don't have good AoE threat, then obviously you can't cast AoE spells, but you might use Crowd Control more.
Are we doomed to have choose between Blizzard and Polymorph? If you could only cast one of those spells in instances, which would you pick?
Perhaps the solution doesn't lie in player abilities, but in how mobs are created. For example, you can't really AoE in Faction Champs because the mobs have a -75% AoE buff.
What if we extended that buff to all elite creatures? The major difference between Elite and normal creatures would be that elites would be very resistant to AoE and normals are not. At the same time, rather than only having elites in an instance, Blizzard could put in more normal creatures, or convert some elites in a pack to normals, with higher health and and attacks.
Right now, pretty much every mob in an instance is elite, which has made that designation lose a lot of its meaning.
So now you get a choice, depending on what mobs you face. If you face elites, you might have to use Crowd Control. If you face normals, you can use AoE. Fights that currently use Crowd Control can have the adds tagged as normal, but with the same stats as now.
It might even allow Blizzard to make more interesting trash packs. Imagine a pack with 5 elites and 10 normals. You might have to separate out the elites so that they can be controlled while the normals are AoE'd down.
The big advantage of this solution is that you don't have to nerf tanks. They still have good AoE threat for when it is necessary, or when AoE would make a fight better. But they can't rely solely on AoE threat, and they have to single-target at appropriate times. In a multi-type fight, a warrior would use Thunderclap to pick up the normals, and Shield Slam to hold the elite.
In many ways, I think this solution, creating a difference between elites and normals regarding AoE, could give us the best of both worlds.