As for the boundaries. . . I associate those with the dwindling of the great forest. This forest presence is in many ways a manifestation of the power of the earth that is able to directly interact with Humanity. The great forest dwindled to small islands, the Old Forest is one of these and the one that is considered the most "alive." Bombadil has chosen to make his abode in the last great bastion of directly manifested earthly power (for another look to the Ents and Fangorn).

As for powerful representations of "other things", I meant this in a mythic manner. In the way that Ares represents war, Poseidon the Ocean, Apollo the son in Greek mythology. Read the Silmarillion for Tolkien's "gods" if you have not already done so. If you have read the SIL, then you should know of what I speak.

However, I am not suggesting that Bombadil is not an enigma. He certainly is and that is why we are so intrigued by him.