Jerome Valeska will have a bigger role this time around, and will feature in more than three episodes.
We will be having a massive team up of supervillains, which David Mazouz has described as the "Justice League of Villains". Cameron Monaghan also teased the team up with a photo from Batman: The Long Halloween showing the villains together. Jervis Tetch/The Mad Hatter is also returning, and Clayface is presumably going to be part of the team up as well. Scarecrow is also coming back, this time with a new actor.
Most exciting of all, Bruce's fear of bats will be introduced this season.
It has been confirmed by David Mazous and Cameron Monaghan that Jerome is not the Joker (which producers have confirmed several times before). According to Mazous, the proper Joker will be introduced soon.
If any of the behind the scenes photos are anything to go by, Harvey Bullock will be teaming up with Jim Gordon again. However it is uncertain if he will be returning to the G.C.P.D., although it seems likely.