I would really, really love a multiplayer version of the campaign/singleplayer mode, with the map, multiple missions, restocking supplies, etc. A lot of us would, actually.
How viable is this?

viewtopic.php?f=19&t=5843
viewtopic.php?f=19&t=5115
viewtopic.php?f=19&t=5149
viewtopic.php?f=18&t=5974
viewtopic.php?f=18&t=5297
viewtopic.php?f=19&t=5012

What does this collection of random links tell us?
1. Actually check to see if someone has already suggested your idea.
2. I'm pretty sure the devs are aware of the concept of multiplayer campaign, most likely before anyone even suggested it.
3. People really, really want multiplayer campaign, and they probably don't care if it's buggy af, aslong as it's possible.

A multiplayer campaign is very likely to be added in the future - I think I'll start putting some more though into the actual implementation when the new netcode is ready. Programming-wise it's not that complicated to add, but there are quite a few game design questions that do need to be sorted out. How respawning works, when and how new crew members are hired, who gets to choose which supplies to buy at the outposts and which route to take, what happens if the sub is irreparably damaged and whatnot. There's already lots of good ideas in those links posted by Esoudiere though! :)