A Fantastic Voyage.
If you're on the lookout for an absolute top-tier couch co-op game for up to 4 players, Lovers in a Dangerous Spacetime is it.

What do you do in this game?
With only a barebones crew of player-controlled characters to manipulate the various weaponry and instruments on-board a tiny ship, navigate and map out unexplored regions of space while attempting to rescue inhabitants that have been scattered throughout the vast reaches. You have a shared health bar to represent ship damage, so basically, the ship is your home and you need to protect it while fending off threats of all shapes and sizes in an effort to secure safe passage.

What makes it so good?
Stages are randomly generated and because the situational needs are always changing, each player will need to shuffle and adapt their responsibilities as well. Every player feels like they're contributing, or able to, at any given moment. No player is locked into playing any one role either, if someone hates the job you're doing and wants to force you out of the captain's seat even though they've only ever looked after the map station, they can. The controls are easy to grasp (pretty much one or two buttons) so it's not really intimidating for players to assume responsibilities beyond what they know and feel comfortable with. It's also very tactile, if you aren't physically controlling an instrument, it's likely not doing anything, so your crew will sometimes need to abandon their posts to ensure they're occupying offensive and defensive stations effectively. Failure is shared and accusations of incompetence are going to be hurled, but in the end, you win or lose together.

What's not to love?
Once you've completed the main campaign and collected all of the unlockables, there isn't a whole lot else to do. The easiest solution would be to add an endurance mode or something where your crew sets out on an endless expedition (area after area with increasing difficulty). Some elements would need to be altered obviously, but it would give the game some much needed replayability. I feel myself wanting to play this all the time, but not really having much of a reason to now that we've finished the main campaign.