I'd love to see a lower megapixel medium RAW option for times you don't need a full 42 megapixels. Canon and Nikon have that and I think it would be useful for those parts of a wedding day (such as reception dancing photos) where you don't need such high res.
Would that be a crop or downsizing? 42mp is overkill for weddings anyway unless the couple wants 40x60 inch prints.
Downsizing. You want to be able to use lenses as intended.
How does that work, exactly? I mean, if you downsize to 1/4 the resolution, you're likely shedding only 25% of data in the sense that you can average out two neighboring green pixels, but you have to record red and blue at the full resolution.
If you downsize to 1/2 resolution and try to do a weighted average of red, blue and green pixels, you're going to be introducing unnecessary artefacts into the image.
To be honest, if you want 12, 14 or 16 bit data, it would be a better idea to do an in-camera RAW->TIFF conversion, then rescaling to the resolution you want and storing this as full color data, but then you're not saving much space (for 10.5 megapixels, you're storing 31.5 million individual color channel data, whereas with 42 megapixels, you're storing 42 million monochrome data).
I can see it working in more crop modes (not just full size and APS-c, but I'd be glad to have cropped RAW when I know I won't need the extra pixels, but could use the extra reach a lot).