The Ivy Pepper that you've now seen for two seasons will be no more. Clare Foley, the young actress that currently plays plant interested Ivy Pepper will not return for the third season due to Gotham's producers recasting.
According to an exclusive in TVLine will be looking for an older actress to play Ivy Pepper aka Poison Ivy. "Casting is underway for an actress in her late teens to assume the now-series regular role of the would-be eco-terrorist." Ivy will also be having a larger role in the series which is also casting several more actors for it's upcoming third season.
Clare Foley has also starred in Netflix's Orange is the New Black as a younger Piper, but outside of Gotham she's likely most known for her roles as Ashley in the 2012 horror film Sinister and as Alice in Jake Gyllenhaal's Southpaw.
In other casting news for Fox's Gotham they are reportedly looking for a Vicki Vale, who was originally portrayed on screen by Kim Basinger in Tim Burton's 1989 Batman. The character is described as "an ambitious young journalist" and in her late twenties. Finally it looks like Dr. Jervis Tetch aka Mad Hatter will be committing haberdashery in the third season.
Ed Brubaker once said of the Mad Hatter "Jervis is obsessive-compulsive, and highly delusional. He's got an immature self-image, so he identifies more with children than adults. Oh and he's a genius, too." If they continue down that line - it looks like the young Bruce Wayne might have a new "friend."