I think that new Warlock card Dark Bargain, the 6 Mana spell that discards 2 random cards to destroy 2 random enemy minions, has potential in a Demon Zoo deck of sorts in place of Doomguard. That sounds like blasphemy I know. Doomguard is so good for 5 Mana when you don't discard anything, provides reach, and can be summoned by Voidcaller. Well Dark Bargain is also really good for 6 Mana. Double Deadly Shot. It's also for Warlock, a class which has poor hard removal but is good at claiming board initiative so it can reasonably reduce the board down to the 2 strongest minions.
I think Dark Bargain is worth considering because there are lots of times where you play Doomguard as the last card in your hand, but attacking face with it doesn't win you the game and instead you have to trade it into a big threat like Dr. Boom, Ancient of War, Mal'ganis, or a taunted Giant. 5 damage isn't enough to kill those threats, and you may even lose your Doomguard. You have to trade other minions into the threats too and end up weakening your board. Dark Bargain however spares your other minions from dying to some big minion.
The other big thing to consider is how Doomguard and Dark Bargain fit into the Voidcaller/Mal'ganis interaction. It often happens that a demon deck will be stuck with both Doomguard and Mal'ganis with a Voidcaller in play. You hope that Mal'ganis gets summoned by the Voidcaller's deathrattle and then you can play Doomguard (which becomes a 7/9) and Life Tap without losing health. You only have a 50% chance of Summoning Mal'ganis in that scenario though. The other outcome is that Doomguard is summoned, and you inefficiently play Mal'ganis for 9 Mana (if you have that much Mana at all). If the Doomguard was instead Dark Bargain in that scenario, you're guaranteed to summon Mal'ganis and then play Dark Bargain to remove the minions that easily threaten Mal'ganis.
It's a meta call. If Face Hunter and Patron Warrior remain rampant, you'll prefer Doomguard in the hopes of killing their hero as fast as possible. If however the meta is full of Ramp Druids and Control decks, Dark Bargain will be better. As a bonus it can destroy Doomsayer when Doomguard alone cannot.
Void Crusher also fills the hard removal role Warlock lacks, and defends against mass Freeze/Doomsayer with it's inspire that works even when frozen. It's inspire can even be used multiple times if not answered. It just looks a bit too unreliable at that mana cost. It can come out of Bane of Doom though, so watch out.