The end of season 3 of The CW's hit series Arrow saw Oliver Queen (Stephen Amell) hanging up his Arrow suit and leaving Starling City alongside Felicity Smoak (Emily Bett Rickards), with Thea (Willa Holland) taking up the mantle of the vigilante Speedy and John Diggle (David Ramsey) to potentially get a mask as well.

While Amell may not be the same brooding vigilante we know when Arrow returns for a fourth season, he's expected to show up in a manner which will reflect the next season's lighter tone. Meanwhile, Arrow's first arch-villain Malcolm Merlyn - who was given the title of Ra’s al Ghul by Oliver at the end of season 3 - will be returning for season 4, as confirmed by actor John Barrowman.

Barrowman spoke to the audience at Amazing Las Vegas Comic Con about what's in store and when production resumes, saying (hat tip to ComicBook.com):

"We start filming in July, season four -- I just finalized my deal about a week ago. I'm very excited about going back, I can't tell you anything because I do know a bit about what's going to happen. But if you love Malcolm Merlyn, if you love Oliver Queen, if you love Thea Queen, if you love the Canary sisters, and I say that with a bit of a twist and a joke in it, you're going to love what's going to happen across the DC world."

Barrowman's remarks point toward the season 3 development which could be the most likely reason for Oliver Queen to return: Thea's resurrection in the Lazarus Pit. Malcolm warned Oliver that there would be unforeseen circumstances, but by the season finale Thea still seemed to be relatively unaffected. Fans of the comic books know that every time a subject descends into the Lazarus Pit, they emerge changed, often suffering from temporary insanity. Thea's side effects are roundly expected to show up early on next season.

The most recent hints from Amell about the next batch of antagonists tease the appearance of former Ra's al Ghul protege Damien Darhk and his mercenary organization H.I.V.E. Darhk was mentioned but never seen during season 3, but with a new Ra's al Ghul running the League of Assassins, it makes sense for the mercenary leader to sense an opportunity to move in and seize control. If Thea/Speedy descends into madness at roughly the same time (which would definitely be inconvenient for our heroes), Malcolm could enlist Oliver's help.

Beyond that, Barrowman's mention of events "across the DC world" could suggest a ramping up of the crossovers within the Arrow/Flash universe, which are due to expand further with Legends of Tomorrow and the recently announced Hawkgirl spinoff. Could the new Ra's al Ghul show up in these other shows as an antagonist? Stay tuned for more details on the interconnected DC television universe as they emerge.

Arrow returns to the CW in the fall of 2015.

Source: ComicBook.com