What if an mFT body used...

Cambridge has a calculator that calculates when lens diffraction starts with a given sensor size and pixel density.

http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm

For a MFT-size sensor,

At 12 MP, diffraction starts at f8.4
At 16 MP, diffraction starts at f7.3
At 36 MP, diffraction starts at f4.9
At 40 MP, diffraction starts at f4.6

For this reason, it makes sense to say that the practical upper limit for pixels for MFT is 12 MP or 16 MP. Even when diffraction starts, you have perhaps one or two stops before it becomes objectionable. A person using an Olympus E-P3 with a 12 MP sensor is still safe at f11.

But it would be a hard sell to market an MFT camera with a 40 MP sensor only for users to discover that diffraction has already started half through the aperture range of the kit zoom lens. The pixel peepers would have a field day.

The diffraction issue explains why "point-and-shoot" cameras with small, high density sensors have such issues with IQ. At 16 MP, diffraction for a camera with a 1/2" sensor starts at f2.7.