Sigma filed a patent for a 25mm f/1.2 lens whose image circle fits the Micro Four Thirds sensor.
It is actually part of a series of lens concepts, most of them being f/1.8 designs. By all accounts, one of them is the current Olympus M.Zuiko 25mm f/1.8 MFT lens. It makes little doubt that Sigma designed the M.Zuiko 75mm f/1.8 as well.
So if the the f/1.2 formula ever sees the light of day, it will most likely come out as another Olympus lens, and not a Sigma DN.