When I transitioned from FF DSLR to MILC, I tried m43, Fuji APSC, and finally settled with Sony A7. For what I shoot (landscapes and travel), Sony gives me the quality I was used to (EOS 6D plus Zeiss 21, for example), in a much smaller package (A7 plus Loxia 21, for example).
In terms of lenses, I think that Fuji X and Sony A7 are the systems to go to. Not to say that m43 does not have a full complement of lenses (it does), but I think it is an overpriced system; once you go to the better cameras and lenses, I can't rule out that I would be paying a lot for a system that has a sensor the 1/4 size of FF, and for expensive f1.2 lenses if I want shallow DOF.