Marvel’s Avengers is a game that Square Enix and Crystal Dynamics certainly want you to keep coming back to. Between the campaign and the War Zone missions, there is a wealth of content to sift through. But it seems that the game depends more on new and rotating missions to extend your playtime rather than replayability.
You can play through several War Zone missions alone or with other players online — these include Faction Missions, Iconic Missions, HARM Room training, and Vaults. These missions are replayable and will have a nice little checkmark below their icon on the War Table. Missions will scale according to the power levels in your Strike Team.
However, a Crystal Dynamics representative confirmed that campaign missions cannot be replayed. This is something that the developers are “exploring for post-launch,” but once you complete a single-player mission in Marvel’s Avengers, there’s no going back. One solution would be to delete your save file and start over again, but that’s a drastic nuclear option.
It could very well be an issue of balance — the campaign missions have an abundance of resources and loot to offer, so perhaps the developers simply don’t want the campaign to become a grinding farm. If you want to make sure that you pick up all of the collectibles through the campaign, then you better keep a sharp eye. And if you are seeking any sort of “new game plus” feature, you might have to wait for an update, if the feature is even coming at all.