Marvel's Avengers is both a single-player and a four-player multiplayer game, but its campaign is a completely single-player experience. That's right, the game's core campaign can't be played with other players. That said, as you progress through the campaign, you'll unlock extra side missions, which is when mulitiplayer comes into play. These missions -- or at least most of them -- are playable by yourself though. In other words, if you want to play the whole game by yourself, you can.
As for the aforementioned side missions, they aren't completely story-less. In fact, they fit into the main campaign and make narrative sense. However, if you don't want to play them, you don't have to. Listening to Crystal Dynamics talk about the game, it sounds more single-player with multiplayer added to it than the other way around, which is great for players who prefer the solitary experience. However, it's unclear what the gameplay loop will be once you beat the campaign. What's going to make players want to come back daily or weekly to play more?
As for why the campaign is limited to one-player is also a bit unclear, but it seems to have to do with the mission design themselves. It's worth pointing out that the footage shown in the game's debut trailer is from one of these missions, not a section that allows for multiplayer.
Marvel's Avengers is in development for PS4, Xbox One, PC, and Google Stadia. Barring any delay, it will release worldwide on May 15, 2020. As for additional ports, Square Enix hasn't mentioned, tease, or even hinted at any.
For more news and media on the game, be sure to take a quick gander at all of our previous coverage of the title by clicking right here. Meanwhile, for more information on the game itself, here's an official story pitch:
"Marvel's Avengers begins at A-Day, where Captain America, Iron Man, the Hulk, Black Widow, and Thor are unveiling a hi-tech Avengers Headquarters in San Francisco — including the reveal of their own helicarrier powered by an experimental energy source. The celebration turns deadly when a catastrophic accident results in massive devastation. Blamed for the tragedy, the Avengers disband. Five years later, with all Super Heroes outlawed and the world in peril, the only hope is to reassemble Earth's Mightiest Heroes."