Your player is exploring and some time has passed since he last interacted with the game world or its objects. After x time passes, a mob is spawned in the general vicinity of the player. The purpose is to create content for the player when they fall into a period where nothing is occurring.
Scenario: If players do not see the mob or if they flee from the mob, you could easily end up with areas dense in mobs.
How would you change/improve the design to remove (other than a despawn timer) or make use of the unused mobs that have spawned over time?
This is an entirely creative exercise and there is no right answer to it. The goal is to discuss ways to build on an idea to make a game mechanic more compelling, more fun or simply work better.
There isn't a "right" or "wrong" way to play, if you want to use a screwdriver to put nails into wood, have at it, simply don't complain when the guy next to you with the hammer is doing it much better and easier. - Allein
"Graphics are often supplied by Engines that (some) MMORPG's are built in" - Spuffyre