I would like to ask a question about function quantities, but I didn't find any adequate tags. If I didn't miss something, it might be useful to introduce a new tag, for example "physical quantities".
"Function quantity" appears to be a concept confined to the Lisp programming language, and it is not widely used in physics. (If it is, then the burden is on you to show otherwise, to be blunt.)
Tags are not generally created for single questions (and indeed if the tag were created for your question and not used anywhere else, it would be automatically deleted by the system after a set period). If you want to argue for the creation of a new tag, then the criterion you need to meet is exhibiting a sufficient number of existing questions for which the tag would apply and for which it would be a useful additional organizing qualifier.
As for the question you mention as a motivation, "I was wondering if there are some people out there, who have further developed and formalized this theory", the page you linked to references the paper An Ontology for Engineering Mathematics, also available here. Google Scholar lists just short of 600 citations to this work. (This kind of follow-the-trail is standard, and you're normally expected to be able to follow it on your own in an academic setting.) If you're having trouble finding something specific within that pool, then that could make an on-topic question, so long as the query is actually about physics.