Onton has a nice sound to it, though I don't really like the wisp idea... it should be something you could reasonably expect to find in chests, given as rewards for quests, etc. Although thinking about it now, it doesn't even have to have a real name or appearance in-game. It's just a number that can be modified through the script... and the variable that controls it is simply $*.
The first worlds created will probably have a big part in creating a cohesive mythology for the universe overall. Looking at things as they are now, those worlds will probably be mostly mine. Or maybe not!
Randomly generated monsters, if put in, would probably involve a whole new RPG-esque 'random encounter' combat system, rather than roaming around, and would be customizable, naturally. But since a lot of the game is meant to be open-ended, people should be able to make peaceful, non-combat worlds if they so choose, and forcing monsters into them, even when unstable, wouldn't be a good idea. I want people who prefer casual exploration games to feel as much at home in the BW universe as fans of more 'gamey' games.
My intent is that the deletion process should be slow enough that it would be basically impossible to lose your world if anyone (whether the builder or players) cared at all about its survival. Perhaps there could be some cosmetic 'warping' effect for a little while before the world is actually destroyed, as a warning (and a hint for travelers to donate to the world's upkeep if they so choose.) But if you are still worried about losing a world, you can simply copy the XML data and save it on your own computer, then upload it again later.
EDIT: I suppose that if a world builder wanted to, they could allow randomly generated elements to be created by the game itself. Maybe players who felt like creating monsters instead of worlds could make them independently and set them loose on the universe, and anyone who checked the 'allow foreign random encounters' option could let them into their own world at random...