They already make pedals just like that. I recently sold some on ebay, because they don't work well. The pedal will twist around under your foot, it's like biking on ice skates (in my experience). I found I'd rather go with a bump from a clipless pedal that has a large platform than go with the skating on ice feeling. There's also a smaller problem in that your seat height isn't quite right any more after putting them on, but that's a problem with most other solutions as well.

I just decided not to switch pedals, but if one wanted to the best solution seemed to me to be the Ezy quick release system -
https://www.youtube.com/watch?v=S1aPm-WqZsQ

You can just switch the pedal out by hand without tools in about 30 seconds.

The new Shimano "light' or whatever mentioned above somewhere looks interesting as well.