Assuming that the One Ring remained out of the control or mastery of another great figure — staying in The Shire, or hidden under the Misty Mountains perhaps, Sauron would be able to utilise all of that power that he had transferred into the Ring.
Almost all of Sauron's power remained available to him whilst the Ring existed and it had no other master I believe it likely that Sauron had the full power of a Maia during the War of the Ring.
[Sauron] had been obliged to let a great part of his own inherent power (a frequent and very significant motive in myth and fairy-story) pass into the One Ring. While he wore it, his power on earth was actually enhanced. But even if he did not wear it, that power existed and was in 'rapport' with himself: he was not 'diminished'. Unless some other seized it and became possessed of it. If that happened, the new possessor could (if sufficently strong and heroic by nature) challenge Sauron, become master of all that he had learned or done since the making of the One Ring, and so overthrow him and usurp his place. This was the essential weakness he had introduced into his situation in his effort (largely unsuccessful) to enslave the Elves, and in his desire to establish a control over the minds and wills of his servants.
-- "To Milton Wadman" (The Letters of JRR Tolkien, Letter 131)
For example, when Elendil and Gil-Galad defeated Sauron in combat Sauron's body wasn't destroyed — he abandoned it. He chose to hide out in the east without any body, as the Ainur do sometimes, waiting to return.
Sauron was for that time vanquished, and he forsook his body, and his spirit fled far away and hid in waste places; and he took no visible shape again for many long years.
-- Of the Rings of Power and the Third Age (The Silmarillion)
The great advantages of wearing the One Ring from Sauron's point of view were twofold:
- It controlled the other Rings of Power, not that it increased the power of Sauron himself — although it acted as a lens and focused his own power.
- He was safe. There would be little difference in Sauron's fate if at the end of the novel, Gandalf or Saruman were in control of the One Ring, rather than the Ring being destroyed — indeed the reason that the fellowship was able to even attempt the trek to Mt Doom was that Sauron never even contemplated anyone wanting to destroy the Ring.
Now the Elves made many rings; but secretly Sauron made One Ring to rule all the others, and their power was bound up with it, to be subject wholly to it and to last only so long as it too should last. And much of the strength and will of Sauron passed into that One Ring; for the power of the Elven-rings was very great, and that which should govern them must be a thing of surpassing potency; and Sauron forged it in the Mountain of Fire in the Land of Shadow. And while he wore the One Ring he could perceive all the things that were done by means of the lesser rings, and he could see and govern the very thoughts of those that wore them.
-- Of the Rings of Power and the Third Age (The Silmarillion)
