Some really good ideas going down there. I haven't entered a competition yet, I considered the dream one but it would have taken time out of other projects and I don't think I would have been very good at it, though I did have a good idea towards the end.
Map from a base, map a theme park, map a working factory all sounds great to me
Nobody has suggested this (that I'm aware of) how about map a rat-map of your immediate surroundings - your computer room, your bedroom or wherever you map. 1mm = 1 hammer unit so you would be running inbetween 10ft cups of tea and climbing giant [everyday object]s. Ladders and low gravity help you get around. On such a large scale you can fit in so much detail that when you stand back things look real. It's a real mapping challenge for VM'ing solids and getting the most detail that YOU can, not so much about what hammer can do.
I'm really liking the idea of this now, shame it's probably not going to get picked!