The CW has exactly 10 hours of oringal programs to air each week during its primetime broadcasting hours. Three of those hours will be occupied by Arrow, The Flash and Legends of Tomorrow. In an effort to keep The CW from being renamed 'The DC'. CW President Mark Pedowitz stated that there will not be a new DC Comics show added to the network anytime soon. Disappointing news if you were somehow hoping that Matt Ryan's Constantine would further expand its migration from NBC to The CW in the form of a new show. For now, you'll just simply have to settle for the con artists of the magic world appearing in cameos. The same can be said for The CW Seed's Vixen, who will be voiced by Megalyn Echikunwoke. Don't expect Echikunwoke to get her own live-action Vixen TV series but perhaps she'll have a cameo appearance.
If The CW is is fixated on not having more than 30% of their original programming be comprised of DC Comics superhero shows then the development of any new spinoff hinges on the success of Legends of Tomorrow and how long Arrow will continue. There's unconfirmed rumors that Arrow would end after the 5-years of flashbacks chronicling Oliver's time on Lian Yu (and elsewhere) were all retold. Again, this is an unconfirmed rumor but one that makes sense. Five seasons would be just enough to get the show into the coveted syndication category.
As for Legends of Tomorrow, if ratings don't hold up, it's unlikely that the show would return for a second season which would potentially open the door for another DC Comics show to replace it. But the failure of Legends of Tomorrow would probably have a negative impact on Arrow and Flash so hopefully the show is a hit and it can continue to build off the momentum already established on The Flash and Arrow.