As far as numbers of megapixels and frame rate are concerned... the real bottleneck is not reading the sensor, it's processing the data afterwards.
For example, we pick on a 60D with a digic4 processor and an 18Mp sensor. It is capable of reading that sensor 60 times per second, but can only process 5.9 frames per second.
And I know this will be controversial, but this is one of those cases where a mirrorless design is superior.... no mirror to shake the camera or to slow down the burst. Even a gopro can read it's 11Mp sensor 240 times per second.... Every design decision has it's plusses and minuses, nothing is perfect for everyone.
You just have to question the usefulness of reading still photos 240 times per second. That is not only video, that is high speed video. Unless you have the literal intention of playing that "video" back on a 240Hz screen at its native rate, the usefulness of such a frame rate for a still camera actually eludes me. Same goes for 60fps. It is one thing to look at 12, 14, 16, maybe 20 fps for a still camera. But at 60fps, and worse at 240fps, one momentary press of the shutter will capture a ton of frames.
Assuming we had image processors that could keep up, a 1/10th second shutter press in AI Servo with a 60fps DSLR would mean you get 6 frames. SIX FRAMES for one tenth of a second. That might get you the best shot up to the moment where an Osprey flexes out its claws in preparation for grabbing a fish....but it would still miss the moment where the fish is in-talon, the moment after it is pulled out of the water. If you hold the shutter down for a full second to get the entire sequence of action...that is SIXTY FRAMES!! I don't even want to think about the issue if we actually had a 240fps frame rate (and an image processing chip that could keep up).
There are bounding limits
of practicality here. Not only is there a limit on how LOW of a frame rate you can really have to get enough frames to capture the right moment of action, but there is also a limit on how HIGH of a frame rate is really practical. I'd say a range between 8fps to 20fps is probably it. I don't think I'd want to wade through more than 20 frames per second at the most, and 12-16 is probably really the sweet spot. I'd also venture that the higher the frame rate, the lower you would really want your sensor resolution to be. Lacl of sufficient memory card and disk space would become a huge problem with ultra high frame rates on high or big megapixel sensors. I like 18mp...it's a good number, especially in FF format. Maybe 22, but you'll start having space issues at that point. Even if ultra high stills frame rates are possible, it blows the bounds of practicality.