While Marvel is currently enjoying the fruits of their beneficial partnership with Sony, there are other characters that the studio still doesn't have access to.
Namely the Fantastic Four and the X-Men, as well as any other affiliated villains and side characters that inhabit those realms closely. Because of that reality, the Russo Brothers will have to make some adjustments to their upcoming Avengers: Infinity War: Parts 1 and 2, as certain characters like Mephisto won't be available to them.
In an interview with Screen Crush, the brothers were asked if they planned on using "existing characters as analogs for very important characters you don't own the rights to?"
"Joe Russo: Yes. You can take the core mythology and apply it to different characters.
Anthony Russo: We don’t feel obligated to do that. We would rather see a new version of a story than a literal interpretation of a story that already existed. That’s part of the fun of it for us. So we lean that way with things. But we always want to take what’s fun and creatively exciting from the books. It’s cherry picking for our own narrative needs."
Captain America: Civil War also had to adjust some plot points in this regard, as characters like Reed Richards and Susan Richards both played a large role in the comic storyline. The Russo's seem to take it all in stride, however, and use it as a means to insert some original beats into a much-loved story.