I'd be surprised if they ever reuse a guest, it seems like a one-and-done deal. But still, God of War being a Sony-exclusive game, it could only have happened on a game that was only on a Sony platform, which is why he was in Broken Destiny on PSP.
Lizardman makes the most absolute sense as an unlockable character, and because we have one more slot, and haven't seen other styles used in-game, Edge Master could be the other one. Two slots on the screen left when we have three DLC, I won't believe those slots are for DLC until it happens.
Amy and Cassandra seem prime for DLC, and the last one could be either a guest or Hwang. That's my conclusion.