I already had that idea some time ago, and I tried to develop a concept to solve the main problem with having that many builders participating in one map: the track is usually built builder after builder, from start to finish.
So, upon the announcement of the project there is a big enthusiasm, but as time goes by, many builders lose motivation or lack the time to build their part of the track. This effect is even stronger when it comes to RPG mapping, as it is more time consuming in general.
If I remember correctly, there have been two major attempts to create a community RPG track. First, there was the planned TMX community track, you can read about the failed attempt over at the TMX forums. And second, there was occam's Bay RPG project "Blue Moon", I haven't heard anything of it for over one year however.
To avoid this from happening, I want to introduce the following concept:
Instead of building the track part after part, it is built in modules. The concept is similar to the one in this track (although that one was built in order)
LINK. Every builder is provided with an identical cubicle and has to fill it with his own style. The modules are then assembled later. That way it is easy to substitute those people who wanted to build a part in the first place but lost motivation. Also, nobody has to wait for others to finish their parts, instead everyone can start building right away. Heck, you could even have a building contest for the best boxes that will eventually make it into the final track.
And for those who think it is boring to have just identical boxes, I also worked out a variation that is meant to bring a bit more diversity into the project. For now, I will just leave this idea for discussion, and if there is any interest, I will provide my idea here later on.