[Neithan]

There are other reasons he could not be a Vala. For one, none of the Valar have the same personallity. Also the Valar, at this time, would not interfere with Middle Earth directly, they sent the Istari and even they had restrictions, and I don't think any of the Valar are going to spend so much of their time there. He could also not be a Maia because "he is his own master" and the Maiar serve the Valar. I gave some reasons above why he could not be an Ainu of any kind, but I will add that Frodo said that the joy in Bombadil's house was like the joy of the Elves but "nearer to the mortal heart". Somehow I think that the Ainur would be even further from the mortal heart.