> What I'm asking is, despite the huge volume of sci-fi we can produce, could there be something two thousand years from now that is practically unimaginable to us?

I think it'd depend on whether we discover new physics. The imaginative gaps you mention were downstream of ignorance of certain physical possibilities. Once it became clear that electrical communication at a distance is possible, people imagined global information networks. Once it became clear that sufficiently energetic fuels were possible, people elaborated on the possibilities of space travel. (Tsiolkovsky was very early! He was sketching O'Neill-style cylindrical space colonies back in 1903!)

Unfortunately, we might not be in store for new physics. So what's left is our failing to appreciate the details of how technologies will develop. Everyone predicted an internet; nobody predicted our internet, not exactly. What will be the impacts of, say, good brain-computer interfaces? Or of clinical immortality? We can imagine them in broad strokes, but we're going to be surprised by the details.