Yesterday (more about the concerts later) Menlo held a lecture/discussion session on Schubert, and one question that was asked the presenter was, what do you think Schubert would have written had he lived longer than his ridiculously abbreviated life?
And the reply was, that while we can't possibly imagine what Mozart would have written had he lived to a reasonable age, Schubert is different. The speaker advanced an idea, which he attributed to a mentor of his, that since Schubert had known for some time that he was dying, his massive outpouring of late works - the string quintet, the last piano sonatas, the late song cycles, and more - were an attempt to squeeze out everything he had in him. You could think of them as memos from the posthumous Schubert - this is what I would have written more of in the future, had I lived. That if he had lived, it might have taken longer to write even these, because he wouldn't have been in such a terrible hurry.
And it occurs to me to contrast this with other composers: Mendelssohn, who also died young. You can hear in his last works - I'm thinking particularly of the string quartet op. 80 - a new direction in his composition; that if he'd lived he would have evolved into a very different composer from the Mendelssohn we think we know. But I hear op. 80 not as a posthumous memo, in the way the Schubert works are postulated, but simply as a road sign indicating the direction Mendelssohn was traveling.
You could say the same of Beethoven, who was not cut off anywhere near so early but could also have easily lived much longer than he did. There was a fourth period looming up in his work, but he never quite got there.