[Intrepid Homoludens]

That's exatly what I imagined: Prisoner of War + Thief + GTA3. Sounds like a fantastic idea, and apealing to all age groups. I have some suggestions.

Day/night cycles - 30 minutes game time equals one day. Should give you enough time to explore and discern the patterns of everyone and everything.

Dynamic weather - this should affect gameplay strategies. Sunny days mean a lot more visitors and the staff would be outside frequently. Rainy days increase exploration because of less visitors and outdoor staff activities. Thunderstorms prevent swinging from trees or you'll get struck by lightning.

Physics - you HAVE to include it, man. Excellent for some puzzles and situations, and throwing things.

graphics engine - the QuakeIII engine should be good for this one (study games like Alice and Jedi Knight II).