How about adding some VERY rare and totally random spawns.
Im suggesting 1 spawn in every playfield, on a random location within a time span of 5 hours-7 days (so no use in camping).
Example: Team grabs a mission from a mission booth and heads out to a mission somewhere. By pure chance we come across a creature that none of us has seen before. As we close to investigate it attacks laying waste to half the team before it lies dead.
Let it drop something worthwhile, and make the mobs tough. Perhaps there could be some mystery surrounding the appearance of these mobs that could be connected to the storyline.
Coming across one of these should be considered pure luck, or just bad luck if it ends up killing you. The point is make it rare and make it worthwhile. Oh, and DONT make it spawn automatically at server resets. A server reset should only reset the random spawn timer.
