A whole new meaning to Organic Mapping.
Here's the biggest challenge I can think of for any level designer to come up with. The insides of a giant creature.
Maybe the adventurers have been eaten by a 5 mile long worm, maybe a druidic order has a temple on the back of a giant desert lizard or it could even be a sub-aquatic city inside some titanic underwater beast. The only rules would be thus:
1. At least 75% of the map must be biological in some way.
2. Artificial structures must be justified.
3. The creature's biology must play by the rules of nature. It has to be able to stay alive in its environment.
4. If you decide to put a civilisation in there you must come up with at least a paragraph of backstory/reason for being there.
5. Whilst the map doesn't need to concern itself with every single part, it must give some idea of the enormous scale of the creature itself.
The best idea will most likely get itself written into a D&D campaign in the near future (which I will post up here for you guys to read/play with)