I suppose they could do what they normally do in the comics and work with him to find uninhabited planets and in the process learn that he is actually very much a good guy and then work with him and his herald(s) to stop another threat. Like what happened during the Annihilation Wave.

Mrriddle0 that would be good in my opinion

Doom gets introduced in an earlier movie, has presence in a few including an FF movie, leads to Avengers event where they think the big bad is Doom, the cliff hanger at end is the real Doom has been imprisoned and the actual big bad is Reed Richards posing as Doom. Because you just gotta do a good guy gone super bad now and Richards is perfect for that, a la Ultimate Uni.

I am gonna say that the most logical choices for the MCU Big Bad are either Doctor Doom or Galactus.

Korvac would be a nice villain and I think it would really boost his popularity

Doctor Doom would be a great choice if there going to include Fantastic Four into the MCU.

^Yeah that isn’t happening, he isn’t even in the MCU as of right now and I doubt that’s going change looking at then recent news

I’m thinking galactus and I think his first appearance in a post credits scene will tie in with that base in space nick fury was in at the end of far from home

Galactus