We don't know what the Marvel universe post-Avengers 4 is going to look like. There are a bunch of assumptions that certain movies will be getting sequels, but there isn't a lot of definitive information. One of those movies that everyone assumes is getting a sequel in Phase 4 is Doctor Strange, and we might have some idea of what that sequel could look like. During a Q&A, writer C. Robert Cargill what a Doctor Strange sequel could look like if he and director Scott Derrickson were asked to come back, according to MovieWeb.
"Nightmare will be the villain. … Marvel movies work in threes. They also work outside of their threes. Some of the better Marvel films have more than one villain in them. Me and Scott have not laid the groundwork for it but what I can say is that I have a feeling that whatever Nightmare is involved with, Baron Mordo, being somebody who considers himself the defender of natural law will have something to do with it."
Nightmare, the ruler of the Dream Dimension, wouldn't be the only villain in this piece. Cargill went on to talk about Baron Mordo and what kind of place he'll have in the Marvel Cinematic Universe. His journey to being a villain was mirrored by Strange's journey to being a hero in the first movie, and they are both still walking along those paths.
"For those of you who have ever read the comics, you know that Baron Mordo has a very particular arc, and that's something we discussed with Chiwetel while working on the first movie. And part of the reason Chiwetel wanted to play Mordo was because he has a definitive ending as a character and it's such a fascinating ending. And I do know that whether we're attached or not, that Marvel plans on working towards that ending and fully realizing Mordo as more than just a cardboard cutout villain, but actually taking him to kind of Loki levels of awesomeness. That's the goal with him."
There is a very good chance that Loki will be one of the bodies left on the ground after the dust settles in either Avengers: Infinity War or Avengers 4, so Marvel would be looking for the next villain that fans might begrudgingly root for. For now we'll have to see what happens to Doctor Strange in Infinity War and wait for some Phase 4 announcements.
Summary: As the Avengers and their allies have continued to protect the world from threats too large for any one hero to handle, a new danger has emerged from the cosmic shadows: Thanos. A despot of intergalactic infamy, his goal is to collect all six Infinity Stones, artifacts of unimaginable power, and use them to inflict his twisted will on all of reality. Everything the Avengers have fought for has led up to this moment – the fate of Earth and existence itself has never been more uncertain.
Avengers: Infinity War, directed by Anthony and Joe Russo, stars Robert Downey Jr., Josh Brolin, Mark Ruffalo, Tom Hiddleston, Chris Evans, Chris Hemsworth, Jeremy Renner, Chris Pratt, Elizabeth Olsen, Sebastian Stan, Benedict Cumberbatch, Paul Bettany, Chadwick Boseman, Benedict Wong, Zoe Saldana, Karen Gillan, Vin Diesel, Dave Bautista, Pom Klementieff, Scarlett Johansson, Don Cheadle, Tom Holland, Bradley Cooper, Sean Gunn, and Anthony Mackie. It will be released on April 27th.