I'll jump in for no good reason, even though I'll never be able to afford a "flagship" camera.
Canon and Nikon could introduce new flagship models with much higher pixel counts right now, and still sell quite a few units. Although it's possible they're just selling down existing stock, it's also possible they've got bigger plans. Since it appears that Canon and Nikon have a "gentleman's agreement" not to step too far ahead of each other, they must both be worried about the competition. This means they'll both take big steps forward together. Seems that MF (i.e. larger than 36x24) is the most logical next step. They would also need new lenses to go with those new bodies. Fortunately, computers make lens design easier now. That might explain the slower introduction of new lenses to the FF lines.
Regarding a potential MF sensor, I'd like to propose a speculative feature and see what the "real pros" on this forum think about it (all three of you). For working studio, landscape or still-life photographers (not action shooters) how would you feel about a "circular" sensor that covers the full image circle illuminated by the lens? In reality, this would be a square sensor that covers that area, but all the corners would be dark all the time. Yes, the ring around the circumference would have distortions, depending on which lens you used. But you, not the camera, would decide which part of the image to keep. Of course, no vertical grip would be required. Yes, the larger sensor with wasted pixels would cost a bit extra, but for a "flagship" camera this is less important, and the lack of vertical grip would partially offset that added cost.
What say you, pros?