Marvel has always been good at finding ways to bring characters back. In the comics, characters who die often come back in one way or another, and it seems as though the movies are headed in the same direction. Loki has defied death a number of times, and we recently saw Gamora brought back into the fold (after dying in Avengers: Infinity War) through a time travel plotline in Endgame. Time travel, and the imminent introduction of the multiverse, could be the key to the MCU bringing back some of the characters they've lost along the way.
Among this crop of characters is a plethora of villains that have remained fan favorites through the years. The best villains often have big personalities, and it's fun to watch a character revel in being bad. The only problem is that MCU villains tend to be one-offs, appearing and being defeated in the same film, and then never seen again. There are exceptions to this rule of course, but a majority of MCU villains seem to be gone for good. However, clever use of time travel and the multiverse could give fans another glimpse of the villains they've loved in the past.
This idea would be most relevant for the villains who were permanently defeated, usually through death. Villains like Michael Keaton's Vulture (who was only put in jail) could easily come back in without the need to break the space-time continuum to do so. However, others are a lot harder to bring back without the assistance of time travel or the multiverse.
We know that Loki is coming back, perhaps acting closer to his villainous self again, as Loki takes place directly after the events of The Avengers. Zemo is also set to return in The Falcon and The Winter Soldier, and Red Skull made a sort of return in Infinity War and Endgame as the keeper of the Soul Stone on Vormir. Clearly, Marvel is open to having big villains return, which begs the question: which of the MCU's villains would it be the most fun to see again?
It would be great to see certain villains return and be given more material to work with. Ronan the Accuser from Guardians of the Galaxy was a villain that fell flat for some, due to the writing of his character, but Lee Pace's performance was so skillful that it made fans wonder how good Ronan could have been with material that matched the quality of the actor. He made a brief appearance in Captain Marvel, but seeing him come back full-force with well-written dialogue and motivations would be really interesting to see.
Hela is another villain, and perhaps more of a fan favorite, that could benefit from more to do. Cate Blanchett is certainly a force in Thor: Ragnarok, and it would be so much fun to see her sink her teeth into the character once again. If Hela came back, it would be great to see her given a little more depth than just "bloodthirsty-goddess-of-death". Making her feel even just slightly more human would make the character all the more interesting. She was such a huge threat in Ragnarok, it would be compelling to see some heroes having to deal with her again, perhaps this time to protect Earth.
Speaking of threats, another villain who could make a return is Ego, the big bad in Guardians of the Galaxy 2. If the MCU's Phase 4 is looking for a villain who poses a huge risk, a living planet might be a good place to start. Bringing Ego back in a way that doesn't feel repetitive would be key, and it would be another opportunity to provide an emotional moment for Starlord. It would also be interesting to see a villain like Ultron return, especially now that Wanda Maximoff's story is expanding. Fans of WandaVision speculated that he might show up in the show (which proved to be false), but perhaps he'll make an appearance in Doctor Strange in the Multiverse of Madness as some sort of emotional obstacle for Wanda (who is set to appear in the movie). This would definitely be possible if the multiverse is involved.
There were a lot of great aspects of Black Panther, and the villain appearances were definitely one of them. Killmonger was a much-beloved villain amongst fans, and it would be great to see him come back in some capacity, and perhaps have a less villainous role and undergo some sort of redemption arc. It would also be great to see Andy Serkis's Ulysses Klaue come back, because it still feels like the character didn't get to show his full potential before he died. Serkis's portrayal was really interesting, and it would be cool to see him given more time to grow with the character and flesh him out.
These are all hypothetical situations, of course, but with the introduction of time travel and the multiverse into the MCU, it's going to be easier than ever for these past villains to reappear in new stories. As long as filmmakers use this sparingly and only bring back old villains if they really know what to do with them (and aren't just using it for fan service), the MCU could have a lot of good villain material to work with. It will be interesting to see how many fan-favorite characters - heroes and villains alike - show up in the next phase of the MCU.