Just as you mentioned, Belial and Butcher (and Diablo) are interesting fights. The Butcher posses strong attack that can be dodged. Same with Belial and Diablo. If you take a look at Azmodan, his Void Circle spawns right under you a lot. His minions are weak and the summoning stone is no treat.
That's just my idea, but perhaps the fight should have more and more void circles spawning, but away from Azmodan, but slowly coming in. You will have less and less room to dodge his fireball and rain of corpse.
His minions need to be stronger, and he needs to summoning stones are too weak.
I think you're close to what Azmodan should be. As general of Hell's armies, his fight should resemble something like a round of DOTA. Waves of weaker enemies constantly spawning in alternating strong and weak waves from Demonic Hellbearers and Hell Rifts in front and behind him, making you wade through them to get close to Azmodan, only to get forced back by the tide. Instead of summoning a ball of fire, portal in some a Siegebreaker or a couple of Butchers, since the storyline says they are not unique enemies. By Act 4, butchers should be fairly easy prey anyway, but definitely would be cool to see again.
Throw in some support troops from the Keep to be spawning behind you to make it feel like it's an actual epic battle, and you've got something that'd be fun to play.