Arrow is currently filming the fifth season finale, and the showerunners have been teasing Oliver teaming up with unuasual allies that most people wouldn't see coming. So who are they?
Deathstroke: Both Stephen and Marc have stated that Manu Bennett is returning as Deathstroke, and pictures were taken of Deathstroke on the Lian Yu set (where the finale is probably taking place), but the actor has continuously diniednthatbhe is coming back, his most recent tweet stating that he is currently filming in Auckland, meaning his unavailable to film the Arrow finale. Is it a simple redirect? I sure hope so.
Captain Boomerang: Not only is Deathstroke spotted on the Lian Yu set, but Oliver is seen not only working with both his mother's killer, but Captain Boomerang as well. The villain was last seen as the main villain of the first Arrowverse crossover before his defeat at the hands of Arrow and the Flash, where he ended up imprisoned on Lian Yu alongside Slade Wilson.
Nyssa al Ghul: Today it was confirmed that Katrina Law will be returning as Nyssa al Ghul in order to face off against her sister Talia. Entertainment Weekly, the first site to confirm it, stated that she will be back for a multiple episode arc. While I hope this to be true, it's only been officially stated that she'll be in the finale.
Black Siren: While it was confirmed a few weeks ago that Katie Cassidy will be returning as fan fave villain Black Siren, it can be stated for sure which side she'll be on. Katie posted a picture on her Instagram story showing her in costume on the Lian Yu set. It one of the leaked pictures from the set, a far shot with some tress in front shows Oliver on the beach with Deathstroke and presumably Captain Boomerang (just kind of a male shaped blue blur), but on the far left is a peek at at a person with a female build in an all black leather outfit that resembles the outfit she wears on Flash and Arrow. So it seems like the femme fatale is on the side of Oliver, but it remains to be seen if she really is and if so, why.
More returning characters: Marc recently stated that even more characters are returning, and I'm guessing theyll be Malcolm Merlyn and Roy Harper, and maybe Ragman or Artemis.
It also remains to be seen where Team Arrow is during all of this, as it seems that Oliver has been forced to assemble a sort of dark Team Arrow (helmet guy - Diggle vs Slade; dude with a projectile weapon - Boomerang and Wild Dog; Black Canary esque - Siren vs Dinah; back up Archer in red and black - Nyssa vs Thea or Roy). But I'm guessing that Prometheus has taken them out of the playing field. My theory is that Prometheus abducted them and placed them all over Lian Yu in spots where Oliver's worse moments of Hell took place, as a way to revisit all the former seasons and really torture Oliver. No matter what, this seems to be an awesome finale.