I feel like after Thanos is beat the Skrulls will be next which will be the "New Avengers" era, which is all but confirmed at this point. After that I think it could be Doctor Doom as the next big baddie after the Skrulls because he hasn't really been done properly yet and when done properly he's a really interesting vilain, which has shown to be an important part of movies now days that audiences love. That would also give them an excuse to introduce the X-Men. And not necessarily after Doctor Doom, but I think i later t will be Galactus as it would also be an excuse to introduce Silver Surfer, since he plays an important role in a lot of big Marvel stories, and maybe the Fantastic Four since they're also really important in a lot of stories, but they're usually only involved with the original Avengers, and I have a feeling by then they'll want to shut it all down, or at least this main story and focus on doing one-shot movies with other characters like Squirrel Girl or Moon Knight, and this would be a good way to end it
So it will be the one powerful guy, Thanos vs the Avengers
Then the alien army of The Skrulls vs The New Avengers
Then the powerful tyrant who also rules an entire country, Doctor Doom vs The New Avengers + The X-Men
Then the intergalactic god, Galactus vs The New Avengers + The X-Men + Silver Surfer + (maybe) the Fantastic Four

Idk what else they'll do in between, like introducing M.O.D.O.K., or the Spider-Verse, ect. I just think those are the main arcs