Ok everyone,
So the theme we choose is "Vapor":
You can interpret this as broadly as you wish, it will be very interesting to see how people adapt that to their game jam design.
While it would seem straight forward to go for volumetric fog effects and such, be warned that it is effects like these that generally don't work well in VR, especially stand alone VR as they can be very GPU intensive. This will be a matter of thinking outside the box. Go for a stylised look, good use of (transparent) textures, particle effects, etc.
Good luck everyone!