Marc did a new interview with IMDB and was asked about Katie being on the show’s and he had this to say about her on Legends of Tomorrow
IMDB: This year all the DC shows have access to this group of floating utility players: John Barrowman, Katie Cassidy, and Wentworth Miller. How will you utilize their characters?
Marc Guggenheim: On “Legends of Tomorrow” Wentworth and John will be members of the Legion of Doom. Katie is a far more interesting question because we actually have a story for Katie on “Legends of Tomorrow” that I am super excited about. It could be the thing I’m most excited about. But it’s early, and we’re not ready to spoil what that’s going to be. I think it could be really, really awesome.