Ah. I didn't see it that way. One way around that is to have a thread for missions where all missions reside after completion. A staff member can merge the newly completed mission thread with the archived mission thread.
The Quest for Life: Archived Missions
The Quest for Life: The Search for the Loving Cup
The Quest for Life: Malack's Journey
Archived Missions is where all threads are combined. Search and Journey would be merged there after the mission is complete. An index in the opening post would link people to the correct spot where each mission begins and missions would show up like chapters in a book.
This keeps the missions in a separate thread until completed but frees up thread numbers after the merge.
EDIT: I see a potential problem though that just now came to mind. I think when the threads merges the posts co-mingle which would make continuity a problem. Darn. I though I had a fix for you.