in NO WAY do I agree with a weapon unlock system. It's a terrible idea for reasons already explained. Even still, single player/linear campaign mode might not be a terrible idea. Here's how I think it could work.
The developer won't need to actually develop a campaign, but instead tweak the way maps can be made so map builders can choose to build either a multiplayer map or a "campaign" map. It will have objective points (hold point A for 60 seconds, capture intel at point B, kill every enemy at point C, etc...) as well as enemies placed by the map maker at various positions. As far as actually playing the map goes, I think a four-player co-op thing might work well, or players can just run the map on their own private server. Still kind of a stretch, but it seems like it might be neat.