I have posted one
game idea for Natal in another thread. It was inspired by Mr Peter Molyneux's demo in E3.
I will try one less 'evil' game here
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Out of body experience (OOBE)
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Since Natal can detect depth, it might be able to create a 3D version of you. It might need some help from MS photosynth technology, nevertheless it can be done. (even the bottom of your feet can be mapped to the virtual you)
To get the proportion right, they might need to reference to something, anyway that is not an issue.
Imagine what you can do with a virtual you.
a) You can roam a 3D location which is as real as the real world. Instead of processing polygon, processor power is used to stitch real photo and predict the outcome if angle changes, height changes etc.
Example you can visit a Egypt Pyramid and have your picture taken outside/inside of the pyramid. (maybe an invisible watermark can be included to help forensic)
Although this is an out of body experience (OOBE), it can be a good pre-visit research software. You can also set marking and have it 'ported' to your winMo phone.
You can mark a RV point for meeting up with another person, even though both of you have never been there. You can set 'rally point' for him or her to follow if needed, direct them all the way from the entrance. And the other person does not need to have a xbox, they just need a PC + Internet connection.
b) Take pictures with another virtual person/celebrity
c) play a horror survivor game like Alan Wake using your virtual self.
This technology can be used by 'evil' but everything can be used for 'evil', so no worry. 'Good' guys/gals will prevail if there are any 'Good' guys/gals left in this world. lol
////I bing on 'Hole in the wall' suggested by OP. To my surprise the 1st result is pointed to a guided canoe adventure tour company