this thread is making me think if the world of traps and attractions thread. it is a great idea, but is grinding along so miserably slowly.
my feeling is that most casual players can and will spend months playing an individual fort, and many of them will depopulate the worlds megabeasts and goblin civ leaders in the process. a check in and check out system sounds pretty good though, so it would still be great as an optional rather then forced thing. ie, the worlds are allways available, however, each one has a list of players who have used it and reuploaded it, with an option to view how many abandoned forts and living/dead adventurers there are in it.
you could, with some smart presentation, have all variations of the one world available on the one page, and when a player wants a world, they can browse to the page that has the one particular world, and have the link for the latest version of the world that has had the most players use it right in front of them, but with a few clicks they could check out how many versions of the world there are, who played in them, and sort by how many forts there are etc. idealy, you could also have a program that read the legends entries and threw up the number of megabeasts and civs, and what races there are present, allowing you to browse and search accordingly.
also, if you gave everyone a profile, you could create a voting system whereby worlds used by players who are perceived to have routinely created epic forts without killing off every megabeast and goblin weaponmaster can be easily found, and worlds used by several such players in succession float to the top of the reccomended list. this is something i wish was on the dwarf map archive, so you could easily find maps or players that other browsers generally recognize as awesome.
it would not be hard to also allow players to upload worlds that are rigidly controlled for succession games and the like, the initial uploader could add a list of players, and the world is only made available for download after the player before them has finished their turn, with the option of the organizer to skip anyone who stops responding, or simply make the world become automatically available again after a set time period with no updates.
anyways, ive ranted more then enough. hope i am coherant, it is late and i am tired. this idea is great.