Yes, it would be nice to know what ‘it’ in this sentence refers to, wouldn’t it?
Yes, the correlate word showed up in French in a previous post here, and the German is ‘Es regnet,’ and maybe you can count Spanish in, because, although ‘Llueve’ suffices, Spanish leaves out even subject pronouns that do have explicit referents most of the time.
One the linguistic types here, though, ought to tell us whether this construction goes back to proto-Indoeuropean. If ‘it’ refers to the supernatural, you’d figure we’d say ‘he’ or ‘she’, in line with all those gods and goddesses formed in our image.
‘It’s’ is, of course, a contraction of the pronoun ‘it’ with the verb ‘is’. I don’t think anybody would use the term ‘reflexive’ for this construction. A reflexive verb phrase is ‘[He] hit himself,’ because what hit (the subject) and what was hit are the same but are separately expressed here.
As to your acting-as-a-noun thesis, it doesn’t really sound all that bad. The English ‘-ing’ verb form is considered a gerund when not part of a compound verb form (a progressive tense). Gerunds or verbals act as either nouns or adjectives, as in ‘Raining is mostly what it does around here,’ or ‘The working man is tired.’ In a progressive-tense verbal construction, as ‘I am working,’ it would seem the ‘-ing’ verb form could be considered an adjective modifying ‘I’. Then, in ‘It is raining,’ ‘raining’ could be considered an predicate adjective modifying the indefinite ‘it’. Then, in English, adjectives can stand for whatever nouns or pronouns they can modify, so ‘raining’ can stand in for whatever ‘it’ is. . .in which case, we don’t have to worry about what ‘it’ is, right? You mean you can’t get with it, just because you don’t know what ‘it’ is?
Ray (It’ll never be the same.)