Here is what I've been thinking about about for I few days now but I'm not sure how it'll be received, so here it goes.

People share one map and then pass it along to another PMC member, each member sends a save along the way and the final product gets posted up on PMC as a collaborative effort. Similar to drawing like this:

where each artist would draw a part of the whole picture.

Steps:

Step 1.
Someone creates and titles the map for initial set up purposes.

Step 2.
Player plays and builds for a set number of 'minecraft days' before passing the map along to the next person

Step 3.
The next member picks up where the last person left off and repeats step 2 until all members have had an agreed upon amount of 'times around'

Step 4:
Final title of the map is decided

Step 5.
Final product is posted

I don't know, it's just a simple idea that I've been tossing around in my head for awhile but I haven't had the chance to get a group together to attempt it. If anyone is interested just PM me.