I know he's been a recurring role in the Smallville series, but how about giving the guy a shot at the big screen. I would go for a different feel for the character that was slightly more faithful to the comics version and I'd do a very condensed origin, but the guy has potential.
I know they were tossing around a movie called "Supermax" that would supposedly have Green Arrow having to escape from a maximum security prison filled with supervillains, but I'm not sure that's the best way to go. For one thing, in prison he obviously wouldn't get his costume or arrows (until maybe the last ten minutes of the film).
And definitely give him Deathstroke as his nemesis in the film. No one else would do.
I know you're thinking this could never work without appearing in a Batman film first, but it could be done. Consider the opening moments of the Birds of Prey series. We saw Batman, the Joker, and Barbara Gordon's infamous crippling shot all in a matter of moments in a blurry flashback. The same could be done to give any backstory necessary without changing the origin too drastically.
Why would this be better than just another Batman film? Well, Nightwing had a more light-hearted personality in costume than Batman. His fighting style was much more acrobatic as well. Now add to this the fact that Christian Bale is still up in the air about a fourth Batman film and you are paving the way to replace him in the fourth film without a reboot. Instead, Dick Grayson takes the mantle with an already-established film history.
Look, we have a Captain America movie set entirely during World War 2 and he flies solo the whole time? We established the CGI for a Human Torch in the Fantastic Four movies, so there's the big hurdle beaten down.
Let's go ahead and fill up the roster. Union Jack and Spitfire would be there, with the regulars of Namor, Torch, Toro, and Bucky. That should be more than enough fodder to make a great movie.
I think the key to this would be skipping the Red Skull as the villain. Instead, give us a straight-up confrontation with Hitler, Master Man, or Baron Zemo.
Another hero from a series that has been rumored for a big-screen role, the Flash is long overdue for a film.
Ryan Reynolds was mentioned for the role before taking the Green Lantern part, and I think Bradley Cooper would be a great second choice. Their personalities would really mesh well for a team-up movie later on down the line if it came to it since Barry Allen and Hal Jordan were such good friends.
The villain here is obvious: Professor Zoom. Most likely he'd be saved for a sequel though, and we'd get Captain Cold. He could give a few good moments in the fight, I suppose. I've never understood how a man moving so fast could have such a running dialogue with his rogues though. They'd carry on complete conversations in what was supposed to be the space of seconds while he's a blur. Go figure.
The Justice League of America
I know this is one that will probably never happen, but if we can get the Avengers I can't see why DC won't budge on this. After crafting the CGI for the Green Lantern film, I can't see anything to stop this.
Yes, the Avengers will be a geekfest for most comic fans, but to me this would be the franchise I'd love to see. The massive fight scene at the end of the film with Batman attacking from the shadows while Green Lantern and the Flash tried finesse, and Superman and Wonder Woman were taking the frontal assault, would be intense.
The villains to face? I don't think the Legion of Doom would be the way to go for the first film. In the old JLA comics they usually had one major villain that was enough to give the team trouble. My nominee would be Darkseid. I know he's in Smallville, but let's put him in the movie and show what a real invasion from Apokolypse would be like.