Deadline is reporting that the CW is developing a spin-off of Arrow and The Flash that would include some of the shows already established characters like The Atom (Brandon Routh), Firestorm (Robbie Amell / Victor Garber), Captain Cold (Wentworth Miller) and possibly Caity Lotz… since her character has died on Arrow she may or may not be the Canary.
All the usually suspects are said to be involved – Greg Berlanti, Andrew Kreisberg and Marc Guggenheim. Kreisberg just joined Berlanti as executive producer and writer on Supergirl so he's already going to be busy.
The report also talks about three major DC comic characters who have not appeared in the series yet.
This project seems to exist outside of the normal pilot cycle and may go straight to series as a mid-season fill-in similar to Agent Carter for Agents of SHIELD.
From they way the report reads it sounds like they may be working on a Brave and The Bold type series that would rotate heroes headlining with one established like Firestorm and pairing with one new comer like say Booster Gold. By doing this, its possible they could also be out of the current time and do an episode with Lotz as Canary teamed up with another character form the past.