The Nazgul would have caught him, Frodo and Sam would been eaten by the fell beasts, fell beasts would get sick, Sauron would be happy because he has the ring back, his orcs would totally overrun Gondor, then the undefended Rohan, then Lórien and Mirkwood, then Ex-Arnor... and everybody (except for the free peoples perhaps) would be happy 😄
Well, how much more than you need to know, Phoe? Sauron would indeed rule Middl-Earth as the free armies of the world could not stand against his full power. The Elven havens had great power but could not stand against Sauron himself, as the books specify. It is pointed out that even without the Ring, Sauron could overcome all opposition- even Tom Bombadil, for those weirdos who like him- and so WITH the Ring his victory would have been so much the faster and more terrible. Gandalf makes reference to a victory by Sauron, if it happened, would be so complete that it would create a darkness that no-one could foresee the end of.
It would be the end of all effectively civilisation but Tolkien does not go into detail of what this would comprise of.
Well, everyone at Minas Tirith would have been slaughtered, so you can forget that lot.
He would totally annihilate every Dunadan he saw- too much hassle from those people.
Some Elves he might take captive. Men he would want to swear allegiance to him. Are there any people or places in particualr you wanted some ideas on?
Men, treacherous as they are, would probably do the same thing as they did in the "normal" fourth age... start Sauron sects 😛
I think Aragorn and Gandalf would definitely die; the Hobbits, maybe, survive, as they're just not as "important" for Sauron as the others (at least if they dont possess the rings anymore)... the last elves would fade, Arwen would die of grief, Elrond would be uncomfortable and leave with the next ship to Valinor, many of the inhabitants of Gondor would probably be imprisoned in Sauron's dungeons as the elves of Nargothrond in the first age ... after a long time it might be like in Núrn - most peoples slaves, exploited by the orcs, hardly enough to eat to survive themselves *thinks of situation in Hithlum after dominion of the easterlings*
erm, I would reckon sauron would be defeated again, one way or another. Perhaps the Valar would actually take their fingers out and do something this time!
Perhaps many main characters would die, I would say Aragorn would die as he was King of Gondor and the main strength for an alliance of men to stand against Sauron....also Eomar as he is king of Gondor..
This would probably be a good strategy for the dark lord as this was he first plan at Minas Tirith and Men would have no leadership without their kings....their pledges/oathw on't mean didley squat and men would flee!
Much things are written in The letters of Tolkien, here you can see some topics: http://users.telerama.com/~taliesen/tolkien/lettersfaq.html
In letter #246 Tolkien brings up what might have happened if Frodo wasn't attacked by Gollum. I can't find much of worth on Google. 🙁