12 members each aside two teams in Avengers: Doomsday, one team battling Dr. Doom on a different earth that’s his conquest and the others fighting against his Doom new ally’s The New Illuminati (Earth-838). If you actually look at the title “ Doomsday” it’s pretty obvious Doom’s invading the different earths across the multiverse by stolen technology from the Watcher. Order to created 'One World Under Doom' known as Battleworld that'd be the setups for Avengers: Secret Wars, that’s gonna be the whole plot with the heroes from mainline universe assembled by TVA preventing Doctor Doom from reaching his goal, if he reaches the godlike powers then he will go after the God of Stories.
Half way through the movie they'll be joined by the Fantastic Four, who play the same sized role as the Guardians played in Infinity War. Half of Thunderbolts will join both Avengers and Fantastic Four. Alongside Wolverine, Captain Peggy, and Deadpool assist as well.
That both team considered would be:
A-Team: Captain America, War Machine, Thor, White Widow (Yelena Belova), Hawkeye (Kate Bishop), She-Hulk, US Agent, Wolverine (if Hugh would wants to came back as Logan for second if not be someone else filed his spot), Spider-Man (who keeps the mask on the whole movie), Ant-man, Stature (Cassie Lang) and Doctor Strange.
B-Team: Captain Peggy, Clea, Captain Marvel, Winter Soldier, Deadpool, Ms. Marvel, White Vision, America Chavez, Invisible Woman, Mister Fantastic, The Thing and Human Torch.
Meanwhile, Smart Hulk leads the operation from the Time Variance Authority Office with help from Ouroboros in the similar role in Avengers: Endgame.
The film would have both teams form quickly to have dealt with facing multiverse threat among their pay grade together, then making up by connection between them all by forcing them to forge dynamics and work together under pressure.