Why not have weak versions of mobs wander around a nest and only around a nest(so people can wander around). Once engaged stronger and stronger versions come out that give more fight skill(that return inside the nest on inactivity). Once you beat the last remaining thing in the nest you can destroy it(or leave it there to repopulate). And if that seems boring add rogues like it is now but are stronger versions of normal creatures but are slow or only attack if attacked. Uniques being the exception which give lots of loot.
edit: Also make creatures feed their young the bodies and the loot gets stored in the "nest".
