If we build a big castle or dungeon - many rooms, many branches, it'd provide a good locale for either PVP combat, or for something like hide and seek (ie: plant several dozen chests throughout the keep, but place the item of value in only one of them - everyone involved searches for it). That and I just think it'd be a fun group project to build such a structure.
Back in late 2011 when I was working with a Machinima studio that never got rolling, we were designing several sets for the movies we had planned in Minecraft It was very excited ,especially with the help of world builder. Give us about ten/twenty minutes to work on something, and we could make a very nice looking tavern or warehouse. I recall we made a very great looking water dungeon that was extremely reminiscient of the Water Temple in Ocarina of Time.
I think if we had the same resources, and a limited number of people were assigned the task of making dungeons with one another, it would bring a much larger atmosphere about on the server. There would have to be restrictions placed on what World Builder can be used for of course. Perhaps keep it to that walled off region that is being speculated about. World Builder would only be used strictly for the purpose of making dungeons, and in special cases, towns that have some sort of special purpose which would normally take far to long to build without extra tools.
It's just an idea right now, but I think with proper management, and the right people doing the construction, a team of World Builder's would highly benefit the activity on this server.
My reasoning for having a limit is so that other players can enjoy not knowing what is inside the giant dungeons they are exploring. They will have to go solely by hints layed out, and their own feeling.