Who do you think Tom Bombadil really was

[Tuor in Gondolin]

I would be more inclined to think that if Bombadil is not a Maia, he is a Vala who, like Tulkas, came after the Ainur first entered Ea, and took up residence on Arda when it was finally formed, perhaps without the knowledge of the other Valar. He may have been planted in Middle-earth by Eru as a potential ace-in-the-hole, so to speak. Well, it's a thought.
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Very interesting. But then who is Bombadil and his mate. And River Woman?
Of course, the maiar seemed to vary in potency (Melkor vs. Sauron) so why not
a powerful maia as an "ace in the hole" or a mole? That sly Eru really plans ahead, eh?