The map pictures and histories of the different saves will be put together. Pretty much, I'll be taking a map coordinate picture with the Isoworld utility of a save that has built something in all 64 map squares. I'll port a screen-capture of that screenshot over to photoshop and stick it next to other coordinate shots as they get completed. People can choose to build more on another person's save, (which I will then update the big map with), or pick "virgin territory" by using my original save and choosing a coordinate that nobody has yet taken. I'll also be taking all the unique histories of each fortress save (from legends mode) and adding them to the world history file that will go with the big map. You'll be able to read all the things that happened to everybody else during that year in their forts across the board. I expect "The dwarven uprising" to be an explosion of stories in those first few in-game years, as more and more people add their fortresses and histories to the big map.
Here's the math I'm looking at: A 33 x 33 square world map has 1089 coordinates and each coordinate has a maximum of 256 (16x16) possible embark squares. (1089 x 256 = 278,784 possible embark squares.) If we took the forum standard of 1 RL week per player turn, even if we all were able to build a 64 x 64 area in 1 week, a traditional succession game would take 1,089 weeks (20 years, 11 months) to completely fill the world with 1 week turns, and that's only if the game kept on plugging along without pause, RL getting in the way, bugs popping up, saves being lost, ect... Interestingly enough, figuring about 5 years per player turn, the world would gain 5,445 years of consecutive player history to its calendar. After only 200 or so turns, the game would crash just from the file sizes - it's a known bug in the longer running forum games.
So a traditional "every person takes a consecutive turn after the others" just isn't going to work. Instead, my thought is that we should do this using multiple embarks at the same time. I've volunteered to "sew together" the big map and the accumulated histories into one place, because that would be one hella of an awesome megaproject that everybody can contribute to whenever they feel like it.
For instance, my plan for my reserved coordinate is to create a chessboard made out of 64, 2x2 embarks, some with 21x21 "empty" squares in either dacite or cinnabar, and some with towers built to look like the pieces. Anybody who takes over my save after I post it will have their contribution added to the save and their history come after mine has ended, while someone who uses the same save I started from will be making history "at the same time" I do, in the combined histories of those saves.
While we won't be able to experience the combined world in adventurer mode (that I know of), we'll still have a megamap and megahistory to show for it.