This sounds fantastic and is something I've broadly thought about for a little while. A little while back someone else linked a concept of what they had done with the idea -- initially a paragraph was visible, and some of the words had a coloured box around them which could be clicked to expand on that term, all the while maintaining a flowing prose. So, I think a slightly different goal to what you describe (it was technical writing I think?), but similar enough. Unfortunately I didn't save it and have previously (frustratingly) spent at least an hour looking for it to no avail, so I'm curious to see what you come up with!