Huh, that idea seems so obvious, now that someone else thought of it and did it. Interlaced video in two dimensions rather than one, with the number of fields potentially adjustable on the fly. Trivial to select the number of fields (and thus the max framerate) as any perfect square. A little more work to select it as any number from 1 to the total pixel count. Though you'd need a monster aperture or a very bright subject to get anything but solid black from a one-pixel image sampled at 25MHz.