Current silicone waffers are big enough. Dalsa already makes x-ray chips that are made of 10x8cm chips and they can be butted up to each other. They already make a 30x24cm panel.I'm going to put on my geek hat for a minute, but before I do let me point out that there are far more qualified people to answer that question. Without going into too much detail, putting more photosites on a small chip is a relatively minor R&D and production expense. By and large, it scales with Moore's Law providing that things like test and measurement are able to keep up. Going to a physically larger sensor requires larger dies, larger wafers, new machines, new technologies and possibly new factories.
What this means in plain language is you will probably see a 200 mp DSLR before you see an 80 mp 6x7, though the latter is fun to think about. As Guy pointed out, large sensors really aren't in the cards until there is a major shift in manufacturing technology away from silicon wafers.
All that would be required is for Fuji to make it's current sensor twice as big and then assemble 4 of them together. It would not be all that expensive.
How about something like 4 D700 sensors. Could be done under $10,000
Nikon is rumored to be coming out with a 24MP FF camera that costs under $ 2,000.
There are now many sensor manufacurers.. the sensor is not the problem.
The problem is two things
Can an MF manufacturer engineer the camera body or in Mamiya's case refresh the RZ sufficiently.
The other problem is that the MF business model is still based on how it started. How much they can gouge the photographer because he is saving on film.