And sometimes, a universe can just balloon completely out of control.
Take mine, for example; take both, or all, of mine.
Originally, my friend and I had started roleplaying in the WoW universe. Now of course, this requires some creativity on the part of any RPer, because the timeline is sketchy and there are questions about biology and evolution and, since it's a game, it is by nature incomplete. There must be a full ecosystem in Quel'thalas, for example, with deer and squirrels or equivalent, but all you get in the game is lynxes and dragonhawks, and then giant spiders down in the southern zones. And by the time of the Cataclysm, the Ghostlands must have been at least partly cleaned up.
I could write pages and pages on Quel'thalas alone, in fact, and reams on the cleansed Plaguelands, the fate of Stratholme/Deatholme, bandits and farms and villages and the resettlement of Hearthglen. But I won't.