If you don't know by now the CW is developing a spin off show which is untitled for now. It's supposed to be a team up show and described as a flash vs arrow episode every episode. I have a few hopes for the roster spots on the team. Hawk Girl and Rip Hunter already being confirmed alongside Victor Steins version of Firestorm, Captain Cold, Heatwave, Ray Palmer and Caity Lotz in some form.
First here are my candidates for future appearances or guest appearances.
This could be the detective role for the team which is a must need since Oliver does not fulfill that on Arrow. He also grounds the team in a way that a Batman like character being Human. He could also be a more behind the scenes type member who is used for stealth and assigning missions.
Manhunter( Kate Spencer)
I realize her character was killed off on Arrow however they could fill the Manhunter role with a sister of Kate's. Just like they did with Sara Lance for Laurel. I think it'd be an interesting dynamic to see how Kate's death triggered her sister's journey to become the Manhunter.
And Speaking of Man Hunter's how about the Martian himself?
Martian Manhunter was an original seven of the justice league. Why hasn't he been cast for the justice league movies? This would be the perfect opportunity for the CW to cast a young actor to fill a huge void in the comic book world. His power set's are amazing and his look would be so impressive if it ever made it to a screen. They already have made Grodd why not Martian Manhunter?!
Now this one's a stretch... Elongated Man!
Elongated man! Who doesn't like Elongated man?! Oh? Never heard of him? Well here's a quick bio of the lesser known stretching hero. He also was mentioned with the list of the people who died because of the particle accelerator. All of the villains who were named have shown up so far as long as Ronnie Raymond(Firestorm) alive. I believe that the characters along with Fire will make an appearance sooner rather than later and this is the perfect platform.
Last but Not least Zatanna!
Honestly I'm surprised how far away Arrow and the Flash are swaying from the Magic Realm. However I think she's the perfect fit to bring it all together. I get that Constantine is running( for now) but it's not connected to the Arrow/Flash verse. She could bring a whole new dynamic to the tv verse that would be amazing on screen!
Honorable Mention: The Huntress
Arrow introduced her and never really explained where she went and what she was doing in her Starling absence this would be the perfect place to show what she has been up to! Plus her and The Question have a pretty hilarious relationship in Justice League Unlimited that I would love to see translated to live action.