Until earlier this year, it seemed like Marvel Studios and Twentieth Century Fox's superhero franchises would continue on with no overlap despite both of them drawing from the same universe of costumed crusaders. Then -- Quicksilver happened.

Following Joss Whedon's revelation that the silver-haired speedster would assemble alongside the Avengers in the team's 2015 "Age of Ultron" sequel, "Days of Future Past" director Bryan Singer unveiled his plan to include the mutant hero/villain in Fox's 2014 X-Men film. Confusing at first, it eventually came to be that the same character will appear in two different franchises, controlled by different studios.

Now, Marvel Studios president of production Kevin Feige has admitted that Quicksilver isn't the only hero who lives in a "gray area."

"There are only a handful of characters that occupy that middle ground," said Feige. "Iron Man is not going to show up over there [at Fox] and Magneto is not going to show up over here [at Marvel]. But there are a few gray points even after many years of negotiations ... and that only happens with a character like Quicksilver, who has been a part of the X-Men, the son of Magneto in those comics, but also a primary Avenger."

With this in mind, here are five other superheroes who might occupy the same rare air as Pietro Maximoff.

Firestar

While Firestar is best known as one of Spider-Man's "Amazing Friends," the character has much stronger comic book ties to the Avengers and the X-Men. Angelica Jones made her in-continuity debut in "Uncanny X-Men" #193 where she enrolled in Emma Frost's Massachusetts Academy. She later made her way to the Avengers following her career-defining stint as a New Warrior. But in the newly released "Amazing X-Men" #1, Firestar can finally add the X-Men to her list of group affiliations, making her another X-Man/Avenger double-threat.

Bonus: Check out this clip of her one and only live-action appearance, at NBC's 1983 "Yummy Awards" TV Special!

Cloak & Dagger

While the duo known as Cloak and Dagger may not have strong ties to the Avengers, they tend to exist in the same territory occupied by the street-level heroes included in Marvel's recent deal with Netflix. These free agents usually remain unaffiliated with any larger Marvel franchise -- that is, unless doing so can goose their profile. The pair, who originally gained their light and dark powers from an experimental drug, was retconned into being mutants during the early '90s X-craze, returning to prominence in the new millennium as they served on Norman Osborn's squad of Dark X-Men. Their connection to the X-Men could be strong enough to align them with Fox, or it could be weak enough to keep them with Marvel.

Namor

Just like Cloak and Dagger, Namor was retroactively granted "first Marvel mutant" status in order to increase his profile. His '90s ongoing series featured the tagline "Marvel's first and mightiest mutant" while his '00s ongoing was subtitled "The First Mutant." But despite his retconned mutanthood, the Sub-Mariner was primarily known for his adventures alongside Captain America in the Invaders and his long stint as a full-fledged Avenger. It wasn't until the past five years that Namor became a leading X-Man -- where he became a major presence alongside heavy-hitters like Cyclops and Magneto. It's entirely possible that Fox could use Marvel's past insistence of Namor's mutanthood against Marvel Studios to get the Sub-Mariner in a Fox film.

Carol Danvers

Captain Marvel's film rights most definitely fall under Marvel Studios' control, a fact buffered by all the rumors circulating around Carol Danvers' eventual inclusion in a Marvel movie. But one can't forget the character's deep, deep ties to Marvel's mutants. Danvers is an old army buddy with Wolverine, and she served as an unofficial member of the team during the classic Brood Saga, which led to the character assuming the cosmic identity Binary and joining up with Cyclops' dad's space pirate outfit, the Starjammers. She's also irrevocably connected to the X-Man Rogue, thanks to an encounter that left the X-Man with Danvers' powerset for almost 20 years. While it's doubtful that Fox could ever use a character called Ms. Marvel or Captain Marvel in their films, one has to wonder if a character called Binary could show up, should the X-Franchise head for the stars.