(CBR) 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 happneed.

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.

Marvel's Firestar is a character stuck between 'X-men' and 'Avengers'

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. 

Marvel's Cloak and Dagger are characters stuck between 'X-men' and 'Avengers'

Copyright © 2014 Comic Book Resources. Reprinted with permission. All rights reserved.