We know that during the events of The Dark Knight Rises, at least some of the inmates of Arkham Asylum slipped their mad house bonds. The fan film Arkham Rising shows how the release of the Arkham inmates might have gone down, and hints at the fate of Joker.
Tito Guillen wrote and directed Arkham Rising. It's great to see Harleen Quinzel hanging around Arkham, apparently one of the few staff members not to flee after the uprising at Black Gate Prison. Hopefully, Guillen can be convinced to make a sequel that indicates how the Joker decided to spend his time in Bane-"liberated" Gotham.