I'm no optics engineer, this is just my understanding of the situation, so someone correct me if I'm wrong:

Micro 4/3 flange distance is really short, which might require some pretty smart optical corrections (if even possible - I'm not sure). Also, teleconverters reduce the amount of light hitting the sensor. There is loss of light from teleconverter optics, in addition to remaining transmitted light being projected over a larger area, giving individual photosites less light to work with. At best, even if one could correct the problem of flange distance with optics, it would make the adapted lenses quite a bit slower. And generally speaking, the more corrective optics are used, the more light loss - with the exception of focal reducers, which just works the opposite way: condensing more light onto the photosites, effectively overcompensating for light transmission loss from the additional optics required.

But let's say for the sake of argument that someone creates a 1.71x teleconverter/lens adapter for the purpose you proposed (which I don't think would even be enough for FF): even if it introduces only the same factor of light loss at best, it would make a 12mm 1.6 (for example) into a 20mm 2.8, with the image characteristics of the 12mm. Realistically there would need to be even more teleconversion, coming with even more light loss. I like an espresso long and dark, not a lens!

Then factor in the issue that a lot of m43 lenses are electronically controlled, the teleconverter/lens adapter would probably have to have a 'smart' version available (a-la metabones smart adapters), which could be unappealingly costly for an adapter that slows your lens down. Unfortunately, due to these caveats, it wouldn't be a very appealing solution.

Sorry to shit on your parade, man.

@tylerknight

This thing exist for many many years and is called teleconverter :-)

No.

An m43 lens does not project a large enough image circle.