While
Marvel's Avengers is both a single-player and multiplayer game, its campaign is solely a single-player experience. The multiplayer aspect comes into play as you unlock side missions by progressing through the game. However, these missions can also be played through in single-player - meaning that the game is entirely playable solo (according to
Comicbook.com).
These side missions will reportedly be totally separate from the game's story, meaning that you don't have to play them at all if you don't want to. While it was thought that the game would be multiplayer for the most part, it seems that it's the other way around. This therefore raises questions like what is there to the game post-campaign? If anything at all.
Something else worth noting about the game being entirely playable in single-player is that; you will be able to play through
Marvel's Avengers without an internet connection. That might not seem like anything special but, in the modern-age with game's like
Ghost Recon: Breakpoint demanding a consistent internet connection in order to play, it's certainly appreciated.
Marvel's Avengers is set to release on May 15th, 2020 for the PlayStation 4, Xbox One, Google Stadia, and PC.