This sounds kind of cool... to me at least XD
My idea is this mission could take place in crater interior and the spawns would be nothing but zus... and thats pretty much it XD you would run through the level and kill all the zus and you win.
I already thought of a mission description: An abnormal amount of zus have been gathering at the crater impact site and have been interfering with our research teams. Silence them.
and when you start the mission you will be greeted by a soldier looking guy and he will say a bunch of stuff: Greetings <character name>, are you familiar with a creature called a zu? Well, recently zus have been gathering in large numbers at the crater impact site and have been attacking our research camps. We have been unable to determine why they are gathering in such large numbers but one thing is certain to us, they must be stopped.
and the winning message could be: Ah, you are back, already the number of zus gathering has started to thin out. This will make our research much easier. Thank you. Please get your reward at the guild counter.
(the reward could be like... 10k meseta or something idk)
and as for the level, you could put a few things in the level that look like buildings and put those scientist people inside the buildings if possible and have then say stuff like: "the zus are not leaving, this is indeed troublesome..." and "What?! is it possible this is the zus mating ground!?" and "Hmm... the abnormal resonations from within the crater seem to be attracting aggrivated fauna to this location...fascinating."
And thats my idea! ^.^ I dont know if it will get anywhere, but build off it if you like, make a spin off with another kind of monster in a different episode or whatever. Maybe it will become a quest in the future =D