I like your idea here. It does seem difficult to me to identify what is and is not extrapolation. Generally, we want things that - somehow - follow. Gollum somehow makes sense in LotR (the corrupting influence of the ring, the durability of hobbits, the segue from an earlier idea to a later more sophisticated one).
Tom is indeed an oddity, and I've read arguments that the books would be as good without him. I'm not sure: to me he's extrapolated from the idea that there is a folkish force to resist the ring's temptations and corruption. I tend to interpret Sauron as the arch-industrialist, and the rings as a deceit - apparent workings of craft but serving the purposes of industry. Prefiguring the moral hazards of consumerism. That is to say, I tend to see Tom as an extrapolation from ideas that run right through LotR.
And so on. Exceptions to the norm are in their way extrapolation. Both acknowledging the norm and offering an authored contrast to it. But not just any contrast... it must make sense somehow.
As I said, I like your idea... "extrapolation" seems like the wrong thing to rule out. Rather I would say that we want the unexpected, even if it is reasonably extrapolated.