Thanos was billed as the top dog in the MCU, and he proved it in Avengers: Infinity War when the Mad Titan walked away victorious. We assume he'll be battling the Avengers once again in Avengers 4, but new rumors indicate that he may not be alone. New leaks from the back of a promotional toy for the movie showed off Thor and Rocket Raccoon with (dope) matching outfits and a description that hinted that there would be a bigger villain than Thanos. The description possibly read that Thor was suiting up to fight "new foes" while Rocket was assembling a new crew to guard the galaxy against "an even greater threat."
If true, then either of these descriptions would have just said "Thanos," if it were him. Thus, these are likely entirely new villains, and while there are practically no clues to work off of, that's not going to stop us from trying. Marvel has no shortage of villains who are above Thanos' power level, and I've cobbled together a list of those who I think either have a genuine shot at being in the movie, or would just be really cool. Check out these six Marvel villains that Avengers 4 could use.
Let's kick off with some hard concept sci-fi, because Avengers: Infinity War cannot be called a grounded film. The Marvel universe is filled with powerful cosmic beings and Eternity is one of those. It's an abstract being that is the embodiment of time. Eternity also represents every living creature in the universe, and if half of the universe just disappeared overnight, then you'd probably be mad. Eternity is an immensely powerful being that could wipe Thanos from existence like it was flicking away some lint. It actually got a little nod in Guardians of the Galaxy Vol. 2, but Eternity might be too hard of a pill to swallow for even die-hard fans.
Speaking of elaborate sci-fi that's hard to digest, how about the Living Tribunal? The Living Tribunal is a being that represents and judges the entire multiverse. It's basically the Supreme Court of cosmic beings and only involves itself in matters when things get real. It often takes the form of a great golden being with three cloaked faces. Like Eternity, the Living Tribunal is not a villain per say, but if it decides to destroy the Earth for the greater good, it will do so without hesitation. Maybe Thanos' actions throw the cosmic balance out of whack, and the Living Tribunal arrives to wipe the slate clean, destroying everything in the process? However, the Living Tribunal might be too powerful a foe to defeat believably.
Doctor Strange's arch-nemesis could be down for Round 2. As the ruler of the Dark Dimension, Dormammu is an insanely powerful being with multiple powers under his belt. He doesn't show any of these off in Doctor Strange, but that's all the more reason for him to take some screentime in Avengers 4. Dormammu has an unquenchable thirst for the total dominion over Earth's dimension and with Doctor Strange gone, Dormammu might never have a better opportunity. Plus, he's a villain we have already been introduced to, so it's less legwork. He's a bit of a longshot though, and without Strange, there's nothing to anchor him to the Avengers.
It doesn't get much worse than the devil, right? Well, Mephisto isn't technically the Biblical Devil, but he's the closest thing the Marvel Universe has to one. He's a demonic ruler of a hellish dimension that makes bargains in exchange for souls, and he's done battle with Doctor Doom, Doctor Strange, and weirdly enough, Spider-Man. He had a big role in the Infinity Gauntlet storyline that partially inspired Infinity War, but he didn't appear in the film, so at least that's some justification for why he could be in Avengers 4. Like Dormammu, he might not be the right fit for Avengers 4, but it's not like Marvel hasn't surprised us before.
The Celestials are a group of powerful beings who helped create life at the dawn of the universe. They usually come in the form of giants in armor, though, Ego the Living Planet proves that they don't always look like that. The Celestials were introduced in Guardians of the Galaxy (using an Infinity Stone, no less) and were referenced in Vol. 2. They have yet to appear in the flesh, and it's unknown if there are any left. However, they could arrive as a sort of final boss for the Avengers, attracted to Earth by Thanos' universe-wiping snap. They would make extremely difficult foes to defeat, but not impossible. The Avengers beat up a bunch of them in a recent comic book storyline.
When it comes to cosmic storylines, Annihilation is high on the list. The story featured Annihilus, a Fantastic Four bad guy, leading a massive army of bug monsters across the universe in an epic bid to conquer everything. It involved a ton of cosmic Marvel characters and even features the death (or at least one of them) of Thanos. It also just so happens that one of the rumored titles for Avengers 4 is Avengers: Annihilation. Fox still owns the rights to Annihilus, so it's doubtful that he shows up, but with some tinkering the Annihilation Wave itself might be up for grabs. Marvel was able to use Quicksilver because of loopholes, so perhaps something similar can be done for the Annihilation Wave. The Avengers fighting another CGI army might not sit well with people who are tired of always seeing that, but it would help make for a grand and epic ending.