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Images leak of Valve's next game, and it's an Overwatch-style hero shooter
(www.eurogamer.net)
Though the way the leaker describes it makes it sound more like a 3rd person MOBA than a hero shooter.
You're going to miss content no matter how you play. It's the nature of the game. Some decisions lock you out of options.
I can't think of anything you'd inherently miss from couch co-op.