[RenegadeLucien]

Just for the record, I've tried to produce a difficulty algorithm primarily based on "distance to last note on each arrow/hand".

I don't know if there's something inherently wrong with this approach or I was just too inexperienced at programming to see it through to a satisfactory completion but I was unable to get to a result that was deemed usable by myself and the difficulty consultants whom I discussed the results with.

On the subject of neural nets, both myself and Trumpet63 have attempted to use neural nets on FFR's song difficulties using extended level stats. Trumpet got his neural net closer than mine (his had a mean difference of 2.4 points from the actual value whereas mine was 4-5 IIRC) but his used several features (such as note color) that could be cheesed by a clever stepfile artist to over/underrepresent the difficulty of their file (ex. if white notes = high diff, throw in a lot of white grace notes that function identically to jumps in practice.)