All Batman: Arkham series fans have wondered...who is our mysterious new character? Sadly, we can't be certain until the game drops next year, but I have gathered some fan theories that speculate who the Knight may be. Hopefully these might tide you over till then.
1. The Joker
Ok, first off is the most obvious choice...BESIDES THE FACT HE'S DEAD! Yes I believe Joker will return in Arkham Knight, but probably as a hallucination or a vision from fear gas, not as the Arkham Knight. NEXT!
I like this theory a lot. Hush's side quest was very unique in Arkham City, with much story left to tell. Warning, Spoilers Ahead! At the end of his side quest, we find out Hush has reconstructed his face to look like Bruce Wayne. Sadly, that is all we see before he leaves us with-out a single drop of closure. His skill set would match the Arkham Knight's, but I have one problem with this theory, the motive. Why would Hush reconstruct his face, just to jump in a suit to kill Batman? Like Joker, I believe we will see Hush again, just not as the Knight.
3. Jason Todd
Ah, one of the two Robins that have not appeared in the Arkham Series. Only problem is that Red Hood is in the game, so there went that theory.
4. Damian Wayne
I wish this theory could be true. I mean it would be possible, but the timing does not match up. Talia al Gaul did mention a night with Bruce in Metropolis, but for Damian to be old enough to be the Arkham Knight, he would have to be at least 20, so another theory bites the dust.
5. Polka Dot Man
Cause why not?
6. A minor character we ignored initially
This is my theory. I believe that the Arkham Knight was in the series before and we just have not suspected it because he or she didn't play a significant role. This seems the most possible to me.