At the outset, I've got to say that I'm very skeptical about Warner Bros.' plans for an interconnected cinematic universe. I don't think that Man of Steel was a good enough film to build such a universe around. If the upcoming Batman V. Superman: Dawn of Justice is as good as DC fans hope it will be, then I'll be more optimistic.
1. The Flash
The Flash is the superhero I'd most like to see in his own film. This is partly due to the fact that I spent most of my childhood reading the classic Bates/Infantino run on the comic, and partly due to the fact that he's such a visually interesting character. It saddens me that Marvel beat Warner Bros. to the punch with TWO versions of Quicksilver, one in the X-men franchise, and one in the upcoming Avengers: Age of Ultron. Warner Bros. is of course producing a TV show and plans to introduce him into their cinematic universe, but given what I've seen so far I am not optimistic about either venture.
2. Captain Atom
Watchmen's Dr. Manhattan is an adaptation of Captain Atom. He is one of DC's acquisitions from the long-extinct Charlton Comics, and he was created by the great Steve Ditko. The history of his powers and abilities is a bit muddled (especially after the New 52), but to make a long story short he taps into a quantum (or nuclear) force that exists outside of the normal space-time continuum. I like the idea of using him to bring aspects of quantum physics into a superhero world. I can see a lot of possibilities there, if the character isn't oversimplified. Watchmen (both the film and the comic book) has of course done this already, but I'd like to see it done with the real Captain Atom.
3. Wonder Woman
I can't figure out why they haven't made a Wonder Woman film. Her background as an Amazon and the mythological origins of Paradise Island offer a more interesting origin story than any other superhero. She's set to appear in Batman V. Superman, but the actress playing her looks nothing like Wonder Woman. The costume is also a letdown. This character, if done right, could be amazing.
4. Shazam/Captain Marvel
Warner Bros. has plans for a Shazam movie, and Dwayne Johnson has already been cast as Black Adam. The script will probably undergo a thousand rewrites and maybe even a few changes of directors before it finally gets made, but it might be good. I think Shazam would work better as a period movie, set in World War II, but I doubt that we'll get to see him fight Captain Nazi.
5. Swamp Thing (the Alan Moore version)
I'm probably only one of three people who bothered to see Return of the Swamp Thing, but the title montage made me hopeful, and the overall movie wasn't that bad. I'd really love to see some of the horrific and metaphysical themes that Moore introduced in the comics make their way onto film. A Swamp Thing movie could be DEEP.
Right now Marvel Studios is a lot like my hometown football team, the Seahawks. They've won the (metaphorical) Superbowl, and now everyone's wondering how long their success can continue. This summer's Guardians of the Galaxy was a massive hit, and next year's Avengers: Age of Ultron is likely to be an even bigger hit. 20th Century Fox's X-men franchise was also reinvigorated by X-men: Days of Future Past, and they plan to release a Fantastic Four reboot next year. Even if Sony's Spider-man franchise is struggling, the other Marvel characters are alive and well.
Jessica Drew, given her history with Hydra, would be a great addition to the next Captain America movie. She is also one of my favorite superheroines, and could be introduced in a stand-alone film. Sony's ownership of Spider-man shouldn't present a problem, especially since Spider-woman never had all that much to do with Spider-man anyway.
2. Doctor Strange
After The Flash, this is probably the character I'd most like to see in a movie. A director has already been assigned to this project, and a lead will be cast soon. Kevin Feige has touted Doctor Strange as the door into the magic side of the Marvel Universe, and there are a lot of visual possibilities inherent in this character.
3. Black Panther
A black superhero, please, and not a sidekick. There are already too many white males in the Marvel cinematic universe. The Black Panther also has an interesting backstory, and could give a larger, international context to the superhero genre.
4. Longshot, Spiral, and Mojo
Longshot has been a longtime favorite of mine. His power is somewhat similar to the Scarlet Witch, in that he is able to manipulate luck or probability. He has also been an X-man, and he hails from Mojoworld, a weird parallel dimension lorded over by the psychopathic Mojo. The three of these characters together could easily hold a movie in their own right, though they could certainly be introduced in one of the X-men films.
5. Namor (the Sub-Mariner)
Namor is so much more interesting than Aquaman. Lately Aquaman has this anti-hero thing going on, but this was just borrowed from Namor, Marvel Comics' original anti-hero. Namor was also a member of The Invaders, a superhero team comprised of Captain America, the original Human Torch, and Namor himself. Their existence predates the Avengers, and would also make a great period film.
Then again, it's possible that 20th Century Fox purchased the rights to Namor along with the Fantastic Four, so perhaps a film featuring him and Captain America is impossible.
1. Nemesis the Warlock
Doing this in live-action would be counterproductive. I'm thinking of something more like the Pixar films, with an art style similar to what Kevin O'Neil did in the comics. It wouldn't be popular by any stretch of the imagination, but I'd love it.
2. Marshal Law
Marshal Law is the one non-Marvel, non-DC character that's crying out for his own movie. In case you're not familiar with the comic, Marshal Law is the product of genetic experimentation, created in the aftermath of a Vietnam-style war which employed superheroes. After most of these superheroes return home as psychopaths, Marshal Law must hunt them down with extreme prejudice. The comic offers a great commentary on American society and its relation to the superhero genre, and would be a terrific counterpoint to the current spate of superhero films. Think Judge Dredd on amphetamines.