Please take note that I'm going to be not including characters like Doctor Strange, Iron Fist and Jessica Jones since they have already been confirmed to appear very soon.
1.
Spider Woman (Jessica Drew)
With Hydra now playing a very large role in the MCU it would be extremely easy for them to introduce her with her origin heavily being apart of Hydra. I understand people who are unaware of the character might think of her as a Spiderman knockoff, but literally with Hydra being to prevelant in what's going on it would be very easy to show that she completely her own character, and the only think she has in common with Spidey is the name pretty much.
2.
Mantis
I understand that Guardians of the Galaxy has yet to even be released, however, there has been news of Marvel even having a sequel planned. Seeing as how the Primadonna is an important member of that team she even has been apart of the Avengers. She has a wide variety of powers, and with the crazy threats the Avengers and Guardians are going to have to deal with they are going to need someone like her. It would even add more of a diversity since she is also Asian. She would be a very good choice for Marvel to choose.
3.
Captain Marvel
I'm honestly not really sure if it's been like 100% confirmed for her to get a movie or not yet. I know they've been searching for a hell of a long time to find an actress suitable to play a Carol so we all do know for sure sooner or later Marvel will bring her in. I would certainly not be shocked if Marvel one day made her the new "face" of Marvel. She is one of Marvel's leading ladies that needs to be introduced in the movies as soon as possible. Also, with her being a character in the series "Alias" she could make an appearance in Jessica Jones' show.
4.
She Hulk
Jennifer is way more of a better Avenger than the Hulk. If you disagree with me then look at the fact that no group ever were forced to send her ass off of the planet for their own safety. With her for the most part being able to control her "hulk" form she would be more of a stable person to have. She could even play a role in the upcoming Netflix Daredevil series since they are both Lawyers. Certainly, if they ever plan on making another Hulk movie the story line of Jennifer becoming She Hulk would certainly be a good idea. (I would still like a Planet Hulk movie though, hmm maybe after?)
5.
Nova
One of the sexist space costumes to date. I know that the Nova Corps will be making an appearance in Guardians of the Galaxy, but nah to Nova sadly, but we could surely expect to see him soon though. He would make an excellent addition to the cosmic side of the MCU.
6.
Spitfire and Union Jack
The reason I put them together is because it would not make sense to really introduce one without the other. With Marvel's Agent Carter being announced it would be literally the perfect chance for them to make an appearance in this show. I suppose Union Jack would be like the first LGBT character in the MCU, right? (correct me if i'm wrong) and we could totally see the transformation of Spitfire being the amazing hero to the fall of becoming a vampire (Which makes her more badass. We all loved that moment of her in Skrull Invasion when she ripped that Skrull's throat out with her teeth. fun times!)
7.
Firestar
People already do know about Angelica from the ancient Spiderman cartoon so it wouldn't be too random to through her in. Even though she is a mutant (that for a minute was trained under Emma Frost, and is now teaching at The Jean Grey Institute). I wonder if she can escape the blackhole of Fox like Scarlet Witch and Quicksilver managed to. I feel she would be a very good role model for cancer patients in the world since she is literally a survivor of cancer, and she also faces the hell of being a college student. People could relate to her easily.
8.
Wasp!!
She was literally planned to be in the Avengers movie but they took her out since she would have probably had the same amount of screen time as Hawkeye, which, is barely none. I am curious if the South Korean actress Kim Soo-Hyun in Age of Ulton will be playing her. I mean, they are filming in South Korea so the character will be important. Also, we need do kind of need the founding Avenger to be introduced like.. really soon. Honestly, with Chris Evan's contract soon ending, and with Robert Downey Jr losing interest in Iron Man I could for sure see Wasp taking a leadership role of the Avengers.
9.
Tigra
One day down the line Marvel will find themselves needing to introduce more younger superheroes, and hey luckily for them they'll have Tigra around to teach these kids. In both Avengers Initiative and Avengers Academy (both very well done titles) she has proved to be a wonderful instructor. You can't really expect most of the current heroes to want to sit around with kids all day? I'm a little nervous at how people would respond to the character before seeing what she is capable of because of the way her costume is just a bikini. Sadly, a lot of people who talk about Scarlet Johanson's performance as Black Widow focuses on her "looks" or mainly how tight the costume is for her ass. A lot of sexist comments are going to be thrown at this character, however, with Marvel frankly not giving a fuck half of the time they should go ahead and introduce her, and prove the "sexy" stereotype wrong. If done correctly, bigots will notice there is more to someone than just their body image.
10.
Cloak and Dagger
Again, with the duo, but hey they are a team. I could certainly see them being introduced very easily in either Agents of Shield or the Runaways movie (whenever they decide to put that project into motion again.) With shows that focused heavily on drugs like Weeds and Breaking Bad and with the huge popularity of superheroes their origin will draw viewers in since Drugs kind of have a strong role in their story. A Cloak and Dagger television show would be wonderful. Maybe if the four upcoming Netflix shows are successful (which they will be) that can inspire Netflix to want to craft this idea into a show?
