Hi,
Pentax is actually making MFD, but that depends in part of them having a heritage. The Pentax 645 system has been widely popular in Japan. There seem to be a lot of very satisfied Pentax 645D/Z users producing excellent images. May be that the lenses are not the best ones ever built, but all proper samples I have seen from those Pentaxes were pretty good.
Fuji has always been a fringe player, with somewhat odd but often excellent products. I would never be surprised if Fuji launched a new and unconventional platform.
Sony is making those 50 MP, 44x33 mm sensors. They could cooperate with Zeiss and relaunch a Contax 645, but I am in the impression that Zeiss is firmly out of the MF business.
Mamiya had some very interesting platforms, paired with some impressive lenses. But, Mamiya is now controlled by Phase One and I don't think we see a lot of innovation from that company.
My guess is that any real innovation in MF may come from either Fuji or Sony. On the other hand, Pentax may also go ahead.
Best regards
Erik
Why would you expect Canon to do that?
I guess if any of the Japanese camera makers would start making MF, they would build their own system from the ground up.