In the Marvel Comics universe, Limbo is a demonic realm periodically inhabited by the kid sister of Colossus, the mutant known as Magik. In the real world, "Limbo" is a pretty good description for where a lot of Marvel's characters are when it comes to film rights. The most obvious right now is Spider-Man. While fans rejoiced to finally see their friendly neighborhood wall-crawler alongside Captain America and Iron Man in Captain America: Civil War, it came at a weird price: Sony and Marvel effectively co-own those rights, with profits from his MCU team-up appearances going to Marvel and profits from his solo movies going to Sony. So, while it's easy enough now to bring Spider-Man himself over to play in the MCU, it's still not entirely clear how much the events in his solo movies will be reflected in the larger Marvel picture.
The Hulk is another interesting case. The character (in his Bruce Banner form) was famously portrayed by Ed Norton in his most recent solo film before Norton was replaced by Mark Ruffalo. He was popular enough in his Avengers appearances that many fans were left wondering why we never got another solo Hulk film. It turns out that while Marvel can legally produce Hulk films, they don't have the distribution rights—those still belong to the original distributor, Universal, who has no interesting in selling them back. Universal wants to make money from any Hulk films made later by Marvel, and Disney-owned Marvel has no interest in making another studio money, so Hulk is relegated to team-ups in the Avengers movies and Thor: Ragnarok.
Finally, there's Quicksilver and Scarlet Witch. Many fans were happily surprised to see these two pop up in Avengers: Age of Ultron. After all, they're mutants—hell, they're Magneto's kids—so it seemed impossible for them to escape Fox's clutches. However, the characters have appeared as Avengers for literally decades, so Marvel had the rights to use them…although those rights aren't exclusive, which is why Quicksilver can be in Age of Ultron while stealing the show in X-Men movies. Moreover, Fox still has a chokehold on anything having to do with mutants, so Marvel can't mention their father Magneto's name in the Avengers movies, or even call them mutants—instead, they're products of Hydra enhancement. As it turns out, Fox has Scarlet Witch-type powers, as they can continue to definitively tell Marvel "no more mutants."