Not much to say.
20) Okay, so let's start with the Builders. It would be interesting to see the movies cross over with the comics and have that universe's Builders trying to destroy earth. However, this is highly unlikely, so only on 20.
19) What better way to make up for a very disappointing villain than to make them the big bad? So yes, Surtur. The problem is, they would need to make a huge personality and power makeover to make him fit to carry on Thanos' legacy.
18) Next on the list is Amatsu-Mikaboshi. The guy ate 99% of Earth-616, so why can't he do the same for earth 199999? However, the creators probably intended the only gods to be the asgardians, so...
17) The issue with Doctor Doom is that marvel doesn't own the rights to him. But, in turn, he would be a nice shift from Thanos, introducing a more earthly, but still magnificently evil villain.
16) What's better than like father, like son in this situation? Thane would be a great successor. He is a torn, unsure youth, and would really make an obscure but great choice for the mantle.
15) While Dormammu bargained with strange, i'm pretty sure he would still have a bone to pick with the sorcerer, and earth in general. So, yes, it would be great if the guy appeared again and attacked earth- himself, this time.
14) Now that Spidey is in the MCU, why not introduce his greatest foe as the new big bad? Not necessarily Green Goblin, but Norman Osborn. I mean, why not? There have been seemingly normal people who have caused a lot of damage in marvel.
13) I'm pretty sure marvel doesn't have the rights to Mojo, either. And while he may be a, say, obscure choice, it would be quite fun to see the guy collect heroes, just as Thanos did stones, and make them fight each other.
12) Another character to who marvel does not own the rights: Galactus. The world eater really deserves better after that non-appearance in FF2. And he could definitely be the next big bad, which would be really cool.
11) I'm not sure whether marvel has the rights to Onslaught, because while he originated with the X-men, he pretty much did damage everywhere. So therefore he would make a great big bad. Maybe he's one of those characters who is owned by marvel and fox.
10) Yes, Vision blew Ultron up. But has that ever kept Ultron down? (Pun not intended) Seriously, Ultron has always found a way to pull himself back. So why not? It could be a great opportunity to advance his character.
9) Look, i honestly hope that marvel owns Annihilus. I REALLY REALLY HOPE! But i can't say for sure. Therefore, he only gets here. I've always found Annihilus to be one of the best "big bad alien guys" and because of that, i hooope...
8) Recently, i read up on old ones and great old ones like Shuma-Gorath and the Dweller-in-Darkness, and it would be awesome if one of these weirdoes suddenly appeared and wreaked havoc in the cinematic universe.
7) There have been many theories about the Secret Invasion commencing in the movies, and that would be an awesome plot for the next few films. So yes, while they, and probably also the Kree are appearing in Captain Marvel, they don't need to be marooned there.
6) I'm sorry if my opinion makes people lose their temper here, but i loved Secret Empire, and i would like it if dear old Cap actually turns out to be an alternate universe hydra counterpart and plots against the avengers.
5) Most people never heard of this guy, but a looney alien who can kill gods? Gorr is one of the best candidates for the future big bad. No, i mean seriously. Have you seen that guy? He's dreadfully evil, and has a relatable backstory! What more?
4) Who better to follow than the devil himself? And please don't wash all the offensive-ness and stuff about Mephisto over me, with a bit of makeover, he could be an awesome big bad, and a different kind of one, for that matter.
3) If you ask me, the avengers have always been continuously annoyed by two villains: Ultron and Kang. And Kang would be an excellent bad guy. Also, introducing Iron Lad and Immortus would be pretty great for the whole thing. So yes, Kang.
2) The avatar of death is the current big bag. So, why not show that life can be bad too? (In the words of Maelstrom, different bad." So now that Warlock is coming, why not introduce Magus with him? Magus really deserves to be the master behind it all.
1) Zemo may have DIVIDED the avengers, but remember who DISASSEMBLED them? The Scarlet Witch. I think she is the ideal big bad for the MCU because she hardly knows what she's doing, and can start all kinds of threats subconsciously, and she also rises from the avenger's ranks. So her insanity may be the cause of Doc Doom or Chthon, she would still make an amazing and compelling villain.
So who do you think would be the best next big bad?
Update: Not sure if this is legitimate, but take a look: https://screenrant.com/best-marvel-villains-fox-mcu-screen-rights-phase-4/
Twenty More Villains
I recently have been flooded over with new ideas, so I'll put them down here.
20) LoveWaffle posted a comment here, which gave me a new candidate idea: Korvac. He is quite a good villain for this, being from the future and different, very different from Thanos.
19) The Beyonder(s). Who knows?
18) If marvel truly gets the right to all the characters owned by Fox, Apocalypse would be a pretty fun candidate to work with, especially because of how Fox did him (not that good). So, yeah!
17) Again, a weird choice- Black Swan, as in Yabbat. She may be different, but she's exactly what a grounded-but-fit-for-big-bad person is.
16) Thanos again. And while this may seem crazy, and i really hope not, Thanos has died several times in the comics and come back, so why not? Maybe because he was already there and that would be boring and repetitive. Yeah.
15) Namor would be an interesting villain, if done right.
14) "Well, we did it my lord Oblivion... death has a new avatar. The end war just begun." Oblivion. Say it with me. Oblivion.
13) So, where did Sam Sterns go? I don't know, but this would be a golden opportunity!
12) Or AIM... what if they get a new leader? Like MODOK? Or Andy? Who knows? Seriously, this would be pretty good!
11) IF the rights go back to marvel, the Phoenix Force would be an excellent antagonist for the heroes to fight, pending the X-Men's arrival.
9) Well, besides Doom and Galactus and Annihilus, another good FF villain would be the obscure Marquis of Death.
8) The squadron supreme/sinister has yet to appear, and who better to battle the heroes than other heroes?
7) If Loki could pull himself back from oblivion, so could Kurse.
6) Morgan Le Fay! How could i have not thought of her on the first list?
5) Again, if you asked me about another villain from the X-Men universe, i would say Donald Pierce. He's a compelling villain, and while i think Logan executed him quite well, he would still be a good villain in marvel.
4) And a weird, weird choice is the Leviathon Mother.
3) Look, i hear people say that not Trevor nor Aldrich were the real Mandarin. If this is true, then the real one is still lurking in the shadows, waiting to strike...
2) D'spayre/Dweller-In-Darkness or so.
1) There is still a mystery in the MCU: Who is Agamotto? Often when he shows himself, he turns out to be some sort of cosmic lunatic. So yeah, here it is!