Thanks to Bernard, I started looking at SKOS. Here are just a few first comments, mostly comparisons to topic maps, but also concerning an interpretation of the structures discussed in the SKOS document. I don't talk about subject identity in this post, but may return to that issue later.
From the SKOS metamodel
SKOS-Core allows you to define concepts and concept schemes.
A concept is any unit of thought that can be defined or described.
A Subject is anything you can talk about.
A Topic is a proxy for a subject: one subject, one topic.
A concept scheme is a collection of concepts.
Sounds like a Category, to me. In fact, later in the SKOS document, they mention the terms fundamental category and fundamental facet.
A concept may have any number of attached labels.
A label is any word, phrase or symbol that can be used to refer to the concept by people.
A concept may have only one preferred label, and any number of alternative labels.
A Topic can have any number of names, each with, or without a Scope.
A Topic can have one baseNameString, stated differently (with Scope, typically in different languages -- multilingual topic names).
SKOS facilitates scoping.
Relationships may be defined between concepts within the same concept scheme. Any such relationship is referred to here as a semantic relation.
Associations may be defined between Topics.
Relationships, as stated in SKOS, sound like the morphisms of category theory. From the perspective of representing, say, a thesarus, I can see the logic in having categories, as one might expect in word senses, where there is a root word which forms a category, capturing all of that word's derivatives within the same category. One can do that directly with the associations of topic maps. There still may be some merit in the category notion.
Mappings may be defined between concepts from different concept schemes. Any such mapping is referred to here as a semantic mapping.
Nothing similar in topic maps. Mappings, as found here sound like the functors of category theory. The SKOS document doesn't appear to go much further with mappings.
I am beginning to suspect, without further research and based solely on a first impression of the opening paragraphs of the SKOS document, that there is a strong category-theoretic underpinning to SKOS. That would be a useful underpinning. I have a strong sense that there is a similar underpinning that can be interpreted in the TMRM.