some good suggestions. I'm thinking the rules will go something like this;
[After the beginning, which I haven't totally figured out yet, suggestions welcome]
Either:
next participant may add between (some fixed number of terrain pieces, haven't decided a good number yet)* And may remove or change some terrain pieces from previous entries* I think we can say; stats may be changed almost freely? This is something that; while certaintly can change the level quite a bit, isn't as personal as a terrain layout.
Now each entry must be a solvable/working level on it's own; it doesn't matter if the level is stupid easy, if you only want to lay a foundation for the next participants to work on, but it must be solvable, with a replay.
Importantly; this number is only a guidline that you should use while making your entry and if you find you must go slightly over or under to complete the idea you have that's okay.
*I think we can use the trust system here, pretty much only change the previous entries to remove a backroute or a thing that's wildly in the way of the solution you're trying to provide. Like Simon said, you can sometimes even make major changes but still keep the original idea of the level there.
For the beginning my only ideas so far are to poll the terrain or randomly select a terrain. Of course now with either Lix or Lemmix you can pretty much mix and match terrains [in Lemmix there is the epic tile set which contains all the originals together...]
I may start a small level myself [myself being the first person in line] Or I might decided to go later because my ideas are exactly flowing at the moment. I could make a poll if people want that.
Also [depending on how many people participate] I think people can go more than once, but not twice in a row.