Interestingly enough while the general idea in this thread seems to be "Hell yeah!", it seems that no one is exact on what should happen, I've read things from "Download it, then upload it when you lose so that other people can travel through with you 'there' and see your old fortress and such crap" to "Download it, upload it, fork if two people get it".
The gist of what I understand supports either of these two ideas (both seem pretty okay IMO):
Idea 1
You search the repository and find one you like, or set one up online (if an option to do so as a member!) and download it, once you download it, it is indeed locked, no one else may download this one until it's 'checked back in' by you.
Passwords can be made for private bloodlines but can be given to others.
Elsewise you can check-out(download) and then check-in(upload), but the problem with this is that unless some sort of check is made to check the time passed in game so that someone can't use the following obvious exploit:
Download file->Upload it directly after->Play file and wait a long time and eventually download the new version (and toss it) and then upload your version, creating unintentional forks, there would need to be some sort of 'check' on the download and then upload, this can be done by CRC32ing a file that you need to download with it, or through other such ways though.
This works great for an 'auto' bloodline, I think this idea would work better if you could do more short-term periods like say download it, play it for a day or two and then have it automatically upload when you next save.
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Idea 2
If it's even friggin' possible something like what was suggested earlier in this thread (multiple times) would be great, for clarity I mean where you have a "World ID", and within that world several fortresses co-existing.
The problem with this is that the only way I see this as being possible is if you allow one succession to go until the end, keep one ahead of the others or something, and it also seems like it would need a lot more than basic coding to mess around with since you'd have to reverse engineer the game.
In any case, this is a great idea no matter which way you attempt on doing it, good luck.