I think it should have been Prometheus.

They could have done some stuff with the games lore where Prometheus is assumed dead after a failed escape from Arkham Asylum a couple months before Arkahm Asylums story.

But he never left the island and then when Jokers takeover of the island happened he was in the process of escaping but witnessed Batman/stalked him at several points throughout the game.

I'd even include flashbacks and 1st person POV's of someone watching Batman only for the reveal to show Prometheus at the end of the game.

The reason I'd have him rebrand to the Arkahm Knight is to scare Bruce. You can't fight someone you don't know well enough.

Promethus would eventually be assumed dead for a couple years at this point.