During a Deadpool Q&A this weekend, it was revealed by writers Paul Wernick and Rhett Reese that they only chose Negasonic Teenage Warhead to be Colossus' "sidekick" because of her name, but surprisingly, the character ended up bringing some complications with her. While Fox obviously own the rights to the mutant, the duo realised they needed to change her powers (in the comic books, she's a psychic), and that ultimately led to them having to ask permission to do so from Marvel Studios.
Rather than just saying yes and moving on, it seems Kevin Feige and company used this to their advantage as a superhero swap took place which resulted in Marvel getting their hands on Ego the Living Planet! That's a character who will play a key role in James Gunn's Guardians of the Galaxy Vol. 2. "Kurt Russell [‘s Ego the Living Planet] in the new ‘Guardians’ movie was the character that Fox swapped with Marvel to [change] Negasonic Teenage Warhead powers," explained Wernick.
It's now hard to wonder which other characters may have been swapped in deals like this and what Marvel would have done had they not been able to get Ego. Star-Lord would have had a different father for starters, but this just shows that the two studios can indeed work together when they want to. Now, it would be nice to think that a similar deal could take place for characters like Galactus and Silver Surfer, but those are unfortunately two Fox probably won't so easily decide to part ways with.