Couldn't Sauron resurface in the future?

[TheLostPilgrim]

Sauron at the time of the Lord of the Rings is lacking the One Ring. It exists, but he does not possess it, and he needs to possess it to have full strength. A good portion of his own power and spirit resides in the Ring itself and was sealed into it when it was create. However, even without possessing the One, he is still a powerful force. Powerful enough to give the West quite a bit of trouble.

What I don't understand is...When the Ring is destroyed, it marks the final and total defeat of Sauron. Why? Yes, he's without the Ring, which had a great portion of his power--But he was without that power, the source of much of his strength, after being defeated by the Last Alliance, and still without the Ring, he posed a great threat to Middle Earth.

While the Ring contains a good chunk of his power, surely Sauron himself still has a great deal of native power of his own remaining in him? Enough to command armies and wreck havoc without possession of the Ring.

While the destruction of the Ring greatly diminished his spirit and potential power (compared to his original self), surely the remaining power would still relatively strong and potent---especially say as compared to the strength of Men? Surely Sauron's spirit, even at it's weakest and with the Ring destroyed, would still, by his nature as a Maiar, be stronger than the spirit of a Man?

Of course, he would not have those things which were created or maintained with the power of the Ring--such as Baradur or the hold over the Nazgul--but couldn't there native power left in him, at least enough to be if not a grave threat, then at least a nuisance to the race of Men, especially as the Elves faded from Middle Earth?

Couldn't his spirit perhaps take "possession" of a weak human and use that vessel to attain power in Later Ages? Perhaps the evil men of modern history, such as Hitler, had the spirit of Sauron within them.

Anyone care to comment on this?