I don’t think this should be done in OSM. There’s a profound number of subgenres of mountain biking, and it’s also incredibly person (based on their skill) and region-specific.
What’s a black diamond (mtb:scale:imba=3) trail in one area could literally be the same as blue square (mtb:scale:imba=2) in another. If the trail has a rating designaged on it, OSM should document that and not try to reassess.
There’s also nothing saying someone can’t try taking a road bike on a downhill MTB trail, or visa versa. This is all user behavior stuff and how-people-use-facilities; outside the scope of OSM.
We as mappers should be documenting the trail as it exists (path, track, etc), document the access permissions (bicycle=yes, foot=yes|no, horse=yes|no, oneway=yes|no, etc), document the trail as it’s signed rating-wise (if it is), and that’s that.
Trying to split things out into subtypes will not go anywhere productive.