My main concern so far with both Doomsday and Secret Wars is how predictable and repetitive the plot could end up feeling, regardless of how good the movies end up being.
Most of the rumors, cause indeed they're nothing more than rumors, hint at this duology being very similar to the Infinity War-Endgame storyline.
The first movie will have, in a way, the villain as protagonist. This I think it's obvious for Doom. Unless, for some reason, Brand New Day includes him in any capacity then Avengers 5 will be the first time we get to meet the character. We need to not only know him, but understand him, just like Thanos.
The movie starts with a divided group of heroes. In Infinity War, the Avengers are broken up.
In Doomsday we get The Avengers and The New Avengers, and it seems they don't get along.
They end up meeting and joining forces with a third group that's more like a family. In IW, the Guardians. In Doomsday, the Fantastic Four.
The heroes divide in two groups. One stays on earth, the other leaves far away (In IW, going to Titan. In Doomsday, as it seems, going to the X-men dimension).
Rumor has it one of these groups will once again be led by Captain America and Black Widow (Steve and Nat back then, Sam and Yelena now).
A young, new-ish hero with a connection to one of the Avengers will tag along (Spider-Man then, Falcon now).
At the start of the movie, the main villain kills off a previous Avengers villain to establish the danger he represents. (Loki back then, rumor has it a variant of Thanos now).
My speculation: A young, naive character is the key for the villain's success and the team on earth will have the task to protect him (Vision then, Franklin now). They get help from a King of a foreign nation. A friendly help from T'Challa back then. Most likely a conditioned help from Namor this time.
The heroes lose the battle on the first movie, partly for not fighting together as a team (IW, they're away from each other and not in good terms. In Doomsday, it's rumored the Avengers and the X-men will be fighting each other in the third act).
The villain reaches his goal (the snap/Battleworld).
The defeated heroes put their differences aside to go on an adventure that will help them fix everything (Time travel in Endgame. I saw a rumor, again just a rumor, that at one point they'd be traveling dimensions and visiting the worlds of non-MCU movies. If it was real at all, I suppose that'd be their road trip this time).
I really hope both me and the rumors going around are very wrong. I wanna be surprised with a more original story. I trust the Russo brothers...at least with Marvel characters.