As people are now living longer, there are more people with various levels of mobility using hiking trails. It would be great to add some trail difficulty data to each trail. It can be as simple as what they use for ski trails or more descriptive by including details like surface type, highest object to step over, lowest object to crouch under, max outslope, max grade, etc. This will allow people with mobility limits to chose trails that they know will be within their abilities. For 85% of the people with mobility disabilities, their disabilities are not visible to others. Also, many trails have restrictions such as no bicycles, no horses, no powered vehicles, etc. This type of data would be useful to help safely keep the right people on the right trails.
I am not sure if we need yet another set of “trail difficulty” tags. There is already a bunch of them, some of them directly related to the aspects you are mentioning.
I also have some doubts about the value of it all. Is anyone really likely to check 15 different aspects of each trail before going on a walk?..
And you never stop the crazy people… In 2003, I have seen someone with a broken leg in a plaster cast with two crutches descending a trail into the Grand Canyon… 
Other people descended on sneakers or slippers and carried no more than a small soda bottle worth of water, or none at all… I can tell you they didn’t look happy once they realized it was a long way up again in +30C…
Do you know the tags that are mentioned on the wiki page about Wheelchair routing ?
This might help you to map some of the things you are interested in.
I’ve also read somewhere that people were interested in pictures of e.g. stiles, because they could only take certain types. They thought that it was too hard to describe them with just a collection of tags.