I'm quite new to forum so I do not know if this would work.
My idea was to make many small scenario's with several exit points leading to other scenario's.
The whole Doom community could make 1 or more scenario's, creating a giant campaign.
When you invite your friends for Doom, you should be able to finish this campaign in 5 hours, but because of the many scenario's that run parallel, you could replay this campaign over and over.
Would this work?