Robbie Amell has been cast in The Flash as the recurring character "Firestorm". In the comic, this character was Felicity Smoak's stepson, so I'm guess he will be reimagined
Despite my hopes and prayer (and sacrifices to assorted pagan deities), Luke Mitchell was not cast as Felicity's new love interest in Arrow. However, he has been cast in the ABC mid-season replacement series, Members Only. According to the press release, he'll be playing Jesse, a rough around the edges guy, who goes to work at an exclusivie country club, and becomes entangled with a wealthy family. This sounds like a total soaper, and I don't usually watch those, but I will tune in for Luke.