(Long time, no see! I was honestly starting to get worried since I haven't seen you around in ~5 months.)
That would be very cool. I've heard of certain standardized tests that do something similar, where as you get progressively more correct answers it will begin asking more challenging things.
Also related: I remember reading an article (ten thousand years ago) where one of the lead game designers of Guitar Hero was describing the way they created each five song "set" that you had to work through before unlocking the next set. It wasn't necessarily monotonically increasing difficulty, but always had some variation. There might be a longer but more relaxed song, something that had some technical sections, and they always considered the final song in the set to be a kind of "boss battle" where the difficulty was much higher than the other four.
That, in turn, reminds me of the "AI director
" in (what is now also a pretty old game) Left 4 Dead. It would enact a kind of dynamic pacing to keep you engaged most of the time, with periodic breaks in the intensity of the zombie attacks.
If we had a reasonably competent random music generator, there could be internal knobs that might be adjusted to do something with the difficulty of the piece along those same lines. A kind of "endless" mode for sight reading practice.