There is always a point in a famous musician's career when a framework for the future is captured on a recording. These recordings are not always the 'breakthrough' albums, or the ones most critically acclaimed. They are just the ones where you can almost hear the gears in the brain turning, just preparing for a brilliant breakthrough. Here are a few of my nominees, and of course, please add more, with explanations:
Brian Eno, Another Green World: This one totally sounds like a mind at work to me, with everything in miniature, genres constantly changing, mixing vocals and instrumental, with a glimmer of the "ambient" to come later....
Jimi Hendrix, Axis: Bold as Love: Pre-suppossed Electric Ladyland. Need I say more?
Pink Floyd, Meddle: The root of all the best stuff they did later.
Feel free to be as opinionated as you wanna be!