This looks like a great project. Can't wait to see how much fun this is for adventurers once we have a few dozen fortresses built.
Please sign me up for a turn!
Regarding timing, with all the changes and new material coming for adventure mode, it's clear that when this project is fully completed it'll be better if it's done in the new release rather than 31.25.
With that said, it may be a while before we can get started, even after the new release. We'll have to wait for DFHack and every other utility we want to use to be updated. The initial releases are frequently buggy, so with a project as grand as this, it may be worth waiting for a few of the first bug fix releases. That will also minimize the potential for bugs to crop up as the save passes through dozens of versions of DF in its year-long trek to greatness. A few such bugs really put a damper on Skyscrapes as it moved from 31.02 to 31.25.
With that in mind, the huge and immediate interest in this project, the long wait times, and the talk of splitting this up into two worlds, I have the following suggestion:
Start a trial/demo succession world right now in 31.25.
It's going to be a long time before any release is as stable as .25, and in a project like this stability is key. It'll also let people start playing right away and flag issues we haven't thought of yet. Oh, and it'll be a fitting legacy for 31, a great last hurrah for future players to explore and get a flavor of this version.
I would start this up myself, but I currently have about 2 hours per week I can dorf, so I can only make the suggestion. If someone agrees with me they should take the initiative and get this moving right away.