Steven?! Never thought of him. Could work!
Yep. He is a singularity. There is nothing in any of Tolkein's contributory or related works that even hints at what or who Bombadil might be. My pet theory has always been that he was a corporeal embodiment of Iluvatar. Very Druidic, very much in keeping with Tokein's mythological and literary roots.
I agree. Mainly by firmly concluding what he is not, I think Bombadil is none other than Iluvatar himself.
Which leaves me wondering about Goldberry. My favorite theory for a while was that they were Aule and Yavanna on vacation (stolen from a web page which backed it up pretty impressively), but upon consideration I canNOT picture Aule singing Bombadil-style.