While introduced in the first season, many have suspected that Cameron Monaghan is playing a younger version of The Joker. While the showrunners have never outright confirmed as such, there have been a multitude of teases that seem to indicate that this is true.
The new trailer titles 'Villains Rising' certainly spells out the fact that Monaghan is playing The Joker. Check out the trailer to see for yourself.
Monaghan himself has teased on social media that he's possibly playing the clown prince of crime. So this is definitely not an outright confirmation but at this point it's looking like this is surely the case.
Gotham Season 2 premieres September 21st.