Sauron seems hell bent on getting back the One Ring. Any word on what he planned to do with it? The question linked as duplicate is about Sauron wearing the ring, which isn't really what I am going for. Of course he's going to wear it. Then what is he going to do with the ring?
The Enemy still lacks one thing to give him strength and knowledge to beat down all resistance, break the last defences, and cover all the lands in a second darkness. He lacks the One Ring.
Gandalf, Shadow of the Past
"A second darkness" is, of course, quite a broad description that could be interpreted in many ways. More detail is given in the essay entitled "Note on motives in the Silmarillion", published in History of Middle-earth 10:
Sauron, however, inherited the 'corruption' of Arda, and only spent his (much more limited) power on the Rings; for it was the creatures of earth, in their minds and wills, that he desired to dominate.
Sauron had never reached this stage of nihilistic madness. He did not object to the existence of the world, so long as he could do what he liked with it. He still had the relics of positive purposes, that descended from the good of the nature in which he began: it had been his virtue (and therefore also the cause of his fall, and of his relapse) that he loved order and coordination, and disliked all confusion and wasteful friction.
But like all minds of this cast, Sauron's love (originally) or (later) mere understanding of other individual intelligences was correspondingly weaker; and though the only real good in, or rational motive for, all this ordering and planning and organization was the good of all inhabitants of Arda (even admitting Sauron's right to be their supreme lord), his 'plans', the idea coming from his own isolated mind, became the sole object of his will, and an end, the End, in itself.
Sauron's ultimate motive is therefore sterile order and domination; that was indeed the reason why he had created the Ring in the first place.