"What's really cool about Ra's coming to the fore is that his presence changes the show," Guggenheim said. "I mean, literally the show will not be the same after Episode 309."
You can see the full interview below.
He added (for those who can't access the video or are outside of the U.S.) that John Barrowman's Malcolm Merlyn has a role to play in the midseason finale, and that he'll have a much more substantial role, including an episode with his own flashback, in 2015.
"I think there's a reason why Malcolm and Ra's have very similar points of view on things," he said, "and who knows? We may even see a little more of a connection between them in future episodes."
Arrow airs on Wednesdays at 8 p.m. ET/PT on The CW.