This list will consist of Marvel properties in no particular order that could become a franchise for the MCU franchise.
Quick Note: There will be no characters that Marvel Studios does not currently have the rights to (i.e. Silver Surfer, Wolverine, or ROM Spaceknight)
The Runaways are a group of reluctant prepubescent friends who discover one night that their parents are an evil organization known as The Pride. Their stories are diverse, charming, relatable, and fun. They could definitely make for a great movie (there was even a script for one at the same time Iron Man was in production, but the movie never saw the light of day).
Dr. Theodore Sallis developed a serum that granted immunity to all known toxic biochemicals, but its mutagenic effects would have transformed people into monsters. While on the run from some AIM goons, he injected himself with the serum and crashed into a magic swamp. It was there that he became the creature known as Man-Thing. Ted Sallis had a straight to DVD release back in 2005, but the movie was less than desirable. A reboot may be in order here.
Eric Brookes is a half-vampire with a vengeful hatred of the creatures of the night. Blade does have a famously recognized cinematic trilogy that is arguably the first good movie to be based on a Marvel Comic. With Wesley Snipes, the man who portrayed Eric Brookes in the movies, being in talks with Marvel Studios for something, It looks like we could see the first case of a film franchise pre-dating the MCU formally join it. Netflix would probably be the best platform for this potential property.
4. Agent Venom
Flash Thompson joined the military and had a mission in Southeast Asia, where he bravely helped his fellow soldiers who were wounded and got through the mission alive, but without his legs. He then volunteered to be the test subject for the U.S. military's Project Rebirth that fused him with the venom symbiote. His origin story would make for a great Netflix series, wouldn't you agree?
Maria de Guadalupe Santiago is the offspring of a man and the ancient goddess Peliali. She has the power to transform into a silver half-rainforest animal. The ones she has been seen using before are jaguar, anaconda, cockatoo, monkey, sloth, puma, cheetah, and crocodile. If she were to join the MCU, it could go either way really. TV Show or Movie.
6. Captain Universe (Tamara Devoux)
Tamara Devoux was a woman who suffered amnesia during a car crash, but also received the power of the Enigma Force, transforming her into the Captain Universe. As Captain Universe, she joined the Avengers in order to maintain the equilibrium of the whole universe. The reason she should have her own movie as opposed to the many other characters who became Captain Universe is because her story is very interesting, The reason the universe chose her was because she had gotten amnesia and lost her daughter, similarly the universe during the point in time she received her powers was also broken.
7. Namor the Sub-Mariner
The half-human half-atlantian king of Atlantis was among Marvel's first superheroes. He would definitely bring a new interesting area to the MCU, and Marvel Studios recently got the distribution rights for the sub-mariner back from Universal, so I think a movie has the potential to happen soon.
8. Ghost Rider
The Spirit of Vengeance has a long history with Marvel dating back to the wild west. I personally, think a Ghost Rider Netflix show set in frontier times would be pretty neat, and perhaps then Johnny Blaze could get a rebooted movie or perhaps the next season of the show would take place in modern times. Either way, Ghost Rider definitely has potential.
9. Black Knight
Yet another character that has a long history with Marvel, I think the same ideas on adapting Ghost Rider could apply here, but taking place in Medieval Times.
Bruce Banner's Lawyer Cousin, Jennifer Walters could be the answer to the long awaited Hulk sequel marvel fans have been asking for, but more importantly, it could introduce She-Hulk, a character with great stories that could translate well to the Movies.
11. Savage Land (Moon Boy, Devil Dinosaur, Ka-Zar, etc.)
There are several great stories Marvel could adapt that link to the Savage Land, (including another potential property that will show up in part 2) and the Three characters mentioned above may be the best ones to adapt into a movie.
For part 2, go here