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.
Not now, at any rate.
Because this post is about universes. My friend and I, then, at one point decided that we wanted to have an alternate universe to play in. Originally, in the AU, Tyrande had somehow been captured by Illidan and taken to the Black Temple, there to be brainwashed into becoming a devoted servant of the Legion - I know Illidan was theoretically opposed to the Legion, but from everything I've seen, in practice he was just another tool of them. He became a demon, and had them working for him in the Temple, after all.
Tyrande, on returning to Azeroth, managed to corrupt Nordrassil on Hyjal, and since it is a world tree, we (well, I, really, and my friend agreed to it) took the liberty of theorizing that the World Tree then began to leak the corruption both backward and forward in time itself. History changed, and kept changing. I have here blatantly stolen some concepts from Terry Pratchett; for more, read all of the Discworld books that feature the History Monks. I don't own all the books just now so I can't reference specific ones.
And then, after things had gotten a little more confusing, we changed the AU again, into the AAU. In this timeline, instead of the Scourge, the Legion had just attacked directly, and gone straight to Quel'Danas, to the Sunwell. No Dead Scar, no Ghostlands; Tranquilien became the new capital, with Silvermoon occupied by demons and green-eyed elves, and the high elf population turned to a seemingly futile resistance, eventually even enlisting the help of the black dragonflight, with Onyxia and Nefarian occasionally showing up in person.
Well, we wanted a universe in which the Dark was stronger than the Light, for experimental purposes. People are different; the Light in everyone is weaker, and the petty evils are stronger all throughout the world, because of the corruption of the world tree.
(We haven't even addressed what the Old Gods are doing in the AU or AAU.)
And this is just the efforts of two people. There's no limit to what a group of people who can work together could produce; this is why I'd like to play the WoW pen&paper version, because there's so much potential from this starting material.
But this, too, is why it's so hard to get away from WoW and start writing my own original fics again. Writing WoW stories, all the heavy work has been done; here is the map of the continents, please ignore the fact that the ends near the planet's poles are rather warmer than they should be; here are the races, their history, art and architecture; here is Draenor, and hints of Argus, and here are some wars, to make things interesting.
I could have sworn I had a point when I started writing this post.