I think dropping the mountaineering in my proposal above might be simpler. This is more thinking out loud than a formal language proposal.
highway=path - what any person able enough to walk and moderately fit is capable of doing.
Class 1 YDS, SAC T1-2, Austria Blue, SWW Yellow, CAI T & E
highway=demanding-path - might need hands for balance, or require some upper body strength (I guess a ladder or mantle would fall in here?) but you’re not climbing rock or have any significant direct exposure. having some experience hiking is highly recommended, but you don’t need to be a climber.
Class 2 YDS, SAC T3, UAII 1, Austria Red, SWW White-red-white, CAI EE
highway=scramble - requires use of hands to pull yourself up terrain. it’s generally simple climbing that is done without a rope but may be uncomfortable to people without technical experience and have fatal consequences. While far simpler than technical climbing, having climbing experience will help greatly. Via Ferrata routes with anything complicated than a ladder or a railing would probably fall into a scramble due to technique and physical strength required?
Class 3 YDS, SAC T4-6, UAII 2, Austria Black, SWW White-blue-white, BMC Grade 1, Schall Scale A, Hüssler Scale K1
(I’m not as familiar with Via Ferrata, I find myself usually skipping using it when in Canada, New Zealand, or Patagonia to just make my own way up rock and it isn’t as common in the US where I’m from.)
I think the best way to structure it would be to just map as many existing regional systems into overlapping categories so there’s something familiar for people to reference and then tweak. What are more European, Asian, or South American trail difficulty rating systems?