Since Arsenal is now officially introduced in the Arrow series, hopefully Teen Titan characters will be introduced as well to weave a Teen Titans spinoff series.
CW once considered producing Raven TV series until it got shelved. If CW continued with the plan, this would have strengthen the DC universe expansion in the CW network. Raven has a very intriguing dark background story that could rival the upcoming Constantine series and could easily blend well with CW's other dark series like Supernatural and Vampire Diaries. The demon's daughter appearing either in the Arrow or Flash series could be a good introduction of the demonic world to the current Arrow/Flash universe. Possible cast: Camilla Belle Routh!!!
CW also considered creating a Wonder Woman series called Amazon but then the idea was later scrapped. Instead of creating a Wonder Woman/Diana origin, CW should have considered Donna Troy origin instead since her origin is far more compelling than Diana. Donna Troy was kidnapped from the Amazons and thrown in the modern world by her alternate evil future self, Dark Angel, and then became a target of assassination of future beings for spawning a possible threat to the future. It would have been an interesting series when Donna eventually discovers her origin and returns back to Paradise Island fighting mythological beings. Of course introducing her in the Arrow/Flash series will open doors to the mythological world. Possible cast: Alexandra Daddario!!!
There have been hints/rumors of Bludhaven in the Arrow series and fans are clamoring for Dick Grayson to appear in the Arrow/Flash series. So the appearance of Dick Grayson/NightWing could surely skyrocket the ratings of both shows. Possible cast: Robbie Amell or Steven Strait!!!
On a side note, The Thorn/Rose Canton should make an appearance on both shows especially on the Flash series since she was the first female antagonist of the Golden Age Flash and the first plant-based villainess before Poison Ivy. Instead of giving her a multiple personality disorder, The Thorn becomes DC's female version of Marvel's Venom, kind of an experimental plant symbiote living within Rose Canton. Possible cast: Karen Gillian!!!