Sure, they could decide to just go for smaller standalone movies now, but if they do decide to go for another big overarching bad guy who will be around for several movies, they'll have to at least be a big enough deal that it'll be believable that the Avengers wouldn't be able to just track them down and stop them in one movie.One thing about the next villain is that I think the next "Big bad" doesn't have to be as "big" as Thanos. People still read the Avengers comic even though they have a lot of adventures aren't life and death for the entire galaxy. He/she/etc also don't need a decade of build up, at least in my opinion.
Yeah, I know Doom's not a huge threat, but he was the only other villain that I could think of who's fought more than one hero or team, and who was part of one of the big comic events. He would definitely need to be set up in an FF movie first, kind of like how Loki was set up in Thor before being the villain in Avengers.Hard pass on Norman Osborne. He's small potatoes in my world (yeah, I know where he's at these days in the comics but I don't care...he should've stayed dead). Doctor Doom would be a good mini-boos (as it were) but I don't think he's quite up there for Big Bad. My choice would be either Galactus or Kang.
I forgot about Kang, he would be another great choice.
What about Annihilus?