I tend to think that what Goldberry
is (meaning, what she is in relation to her presentation in the published version of LotR) is much the same as Tom Bombadil, an enigmatic creation that seems rather a "holdover" from the time that Tolkien was still thinking of LotR as a "Hobbit sequel." Their general nature feels more akin to the world we see in TH than it does to the world we see in LotR.
Now, that said, I sometimes wonder if she isn't also a bit of a holdover from other ideas Tolkien had once entertained, then discarded. From what we see of some of his earlier versions of
The Silmarillion in the HoME books, the Valar/Ainur we see in what we think of as TS aren't the same as what Tolkien had once conceived. At some point, they were more "classic mythology" rather than "angelic" in nature. Even after Tolkien started referring to the Powers as the Valar rather than "the gods," he conceived of the Maiar as the Valarindi, the Children of the Valar; early versions of Eonwe and Ilmare (called, I believe Fionwe and Erinti) were the offspring of Manwe and Varda. I'm not saying that Goldberry (and possibly Tom) was ever another child of the Valar in Tolkien's mind, since no evidence of this exists, but in pure speculation, a character who appears to be more than human, referred to as the "River-daughter" could have (at some time in the evolution of Tolkien's created world) been indeed the daughter of a greater power, like Ulmo, or perhaps even Uinen (after all, if the Valar could reproduce, why would their children not also have this capacity?). Given the scope of what Tolkien was creating, I can well imagine that there were many, many more possibilities he devised, then discarded; and I think that it's possible that some aspects of these myriad variations might have crept into the final work -- some intentionally, others not so, just bits of that bigger tapestry he liked for some reason and didn't want to wholly abandon (rather like Tom himself, I suspect).
Just muddying the waters (so to speak)...
