I think I have put my thumb on the difference between fannish speculation and serious literary criticism of a work.
The difference is that lit crit not only starts with what the text suggests, it goes back and sees if there is evidence of it. The fans who speculate that the Patrician in the first Discworld books was really Snapcase, because he fits the later descriptions of him much better than Vetinari does, have actually come up with a more coherent history than Pratchett has; he attributes it to his having been a less competent writer at the time.
Similarly, speculating about Romeo and Juliet and what would have happened if either or both of them had realized their fundamental problem was living in a nest of vipers, and the only escape being getting out of there -- all very well, but not literary criticism, because nowhere in the play does the possibility of voluntarily leaving arise. (It can be very hard to rip up your roots, and at that time, immigration wasn't really present as an intellectual possibility.)
To which one might state that much of what is now put out by English departments, particularly as deconstruction, would therefore be fannish speculation.
To which I state, feature, not bug.