When I was in University in the 70's, I worked part time for the Earth Sciences dept and developed portable Seismic equipment using geophones for accelerometers. In a room of the same size I could flex my knees on the concrete floor and peg the needle on the concrete floor while the building HVAC motors caused some some rumble at lower levels. We could see a deer walking down the road 100m away and get impulses.
With the right notch filters for ambient resonances, you can detect movement for the whole room. To detect silent motion, you might consider standard IR motion detectors with narrow beam width. Or create a grid of remote control emitters from one controller and a grid of detectors to detect motion , just above the floor level. Each emitter and detector must be recessed for very narrow beamwidth with no overlap. THere may be some deadspots. For static heat sensing, use PIR sensors.
What resolution do you need?
For Wii games they have a touch sensitive sheet of plastic 1.5 x2m that interfaces for dance games but only 1 person.