Make a kind of area that is for a new mob.
The new mob shall be called the Anitary. (Animal + Military) So these should be somewhere near the city, with a HUGE block of area that is closed off and is impossible to build in. SO there will be a new feature that has events or something, like, every day or so there will be an Anitary attack, or a anitary defence, so players make a border around the big space and try to make the anitary troops not get out of the area. And stuff like that. YES it will take a LONG time to script and make, and YES this probably wont be implemented, but YES i reckon this could be an awesome idea.
This is actually one good idea.
ONE OF THE BEST IDEAS EVER