My brother had a very good theory on who the Arkham Knight is.

[SPOILERS ALERT]

My brother thinks that the Arkham Knight is... Batman. As we know, Bruce is messed up after the events of Arkham City, involving Joker's and Talia's death. With Scarecrow being the main villain, we will most certainly see hallucinations involving Bruce's parents and even the Joker. Then the Arkham Knight comes. An "anti-Batman" carrying Arkham's name and trying to stop the Dark Knight represents Bruce's guilt and fear and he must battle himself to coup with the guilt.

It's just a theory my brother came up with, but I think it's better than the Arkham Knight being Hush or Joker's clone/son.