My line idea was, say ya had a bridge. On the side of the bridge would be a blue line, not big, but 64 x 64 square and length of the bridge...then the line would change with the flow of the map, still 64 x 64, the length going up stairs (along a wall), over some rocks, thru a wall, exit out the other side, then continue thru your whole map. The line wouldn't always need to be solid...it could pass thru the player's path from wall to wall (a laser connecting the two lines), but the line would always be visible or "followable" thru the map...and then continue on into the next author's map, same size, but perhaps a different color.
You wouldn't need to build your map around the line, but the line would intertwine thru the map just to keep the theme of the rest of the maps.
And it wouldn't need to be a focal point. It could go from point A to point B over the whole roof of the map, or it could be a focal point, where it would make a center-piece of the one map or go down with a waterfall..or the waterfall would go through it..it needn't be just one line. The line could break into many and even make a cage...but it would need to go back to being one solid line as it exits the map and goes onto the next author.
That would keep the theme of the map and let us all use our own talents and uniqueness to tie the maps together.
Anyways, I like the elemental theme too, but there was a contest here not long ago where people needed to make arena maps with 2 opposing themes...I think we'd be redoing that contest if we all had different elements (fire, water, etc) in our maps.
Also, I volunteer to do the weapon placement and spawns if ya want.