As comics nerds know, the timeline for Gotham is a little off from the comics. A few characters are older than they should be, some are younger, and so on. But showing a young Robin would be a little much even for this show, so instead we’ll witness Robin’s conceptio-er, we’ll see Robin’s parents meeting.
According to an interview Entertainment Weekly conducted with showrunner Bruno Heller, we’ll see Robin’s parents meet for the first time, presumably forming the Flying Graysons. Also coming are a few supervillains as children; Poison Ivy, who we last saw for five seconds in the pilot, is coming back, and we’ll be meeting a middle-school Scarecrow and learning about his difficult relationship with his dad. We’re just assuming right now Bruce will beat them both up using his dad’s watch as brass knuckles.
As for Harley Quinn, she may be coming but she’s not in the first season. Heller’s explanation was fairly straightforward:
That aspect of the show—which characters to use and when — is a source of constant discussion. And that may well have been an issue that came up and was dropped. We haven’t got Harley Quinn in it. Riddler’s girlfriend is coming up. And Harley Quinn is definitely planned for later on, but so far no.
That actually makes a lot of sense. The show at first insisted there’d be a new candidate for the Joker every episode, but it more or less dropped that idea after the first few episodes, even though Heller insists they’re bringing it back. Harley’s pretty much entirely defined by loving somebody who doesn’t love her, and without that, you’ve just basically got an overly limber psychiatrist.
Furthermore, the show doesn’t really need the Joker. It’s built a sprawling story of intrigue and manipulation centered on the Gotham City mobs and the real motivations behind the death of the future Batman’s parents. A laughing madman would arguably be a distraction. Unless he breaks up that stupid Barbara/Montoya plotline. Then by all means! Bring in the Joker!