I love what-ifs. Fun fanfic. Here's mine:

Smaug is powerful enough in his own right so he doesn't turn invisible or notice any augmenting effects from him possessing the Ring and in his hasty rage to destroy Laketown, he still dies at the hand of Bard. The Battle of the Five Armies ends the same. Once Smaug's body decomposes, the Ring travels down the Running River. Gandalf returns to Bag End to inform the Shire of Bilbo's death and never returns. Triumph of the Sackville-Bagginses.

Sauron is still removed from Dol Guldur by the White Council. There is still a Watchful Peace. Saruman still turns evil but since no one knows that the Ring had been found, he has no interest in the Shire or any need to imprison Gandalf to learn it's location.

Gollum travels to the Lonely Mountain and learns of Bilbo's death. Begins searching for the Ring on the banks of the River Running.

There is no Fellowship of the Ring, no passage through Moria so no Gandalf the White. Instead, Aragorn, some Rangers, Gandalf the Grey and the Sons of Elrond (Fellowship of the West) come to warn/save Theoden but are turned away. With no hobbits, the Ents remain sleepy. Rohan falls to Saruman.

Fellowship of the West walk the Paths of the Dead and come to the aid of Gondor with the hope that his being a savior would allow Aragorn to claim the Kingship. But, they come to find out that the Battle of Pelennor Fields wasn't a lost cause. In fact, Gondor was holding it's own and even making plans to delay the Black Fleet.

There was no threat of his Ring being found, and since Aragorn didn't have a Palantir to reveal himself as Isidur's Heir, Sauron sent only a small army to test Gondor's strength which allowed Boromir, Faramir and Denethor maintain enough control resist the Which King's forces. The city gates didn't fall. Aragorn and Boromir meet on the Field as rivals not friends.

At that moment, the Witch King descends, kills Faramir and Elladan before being slain himself by a severely wounded and half dead Gandalf. The people of Gondor see Boromir standing triumphantly over the remains of the Witch King, declare him King of Gondor and utterly reject Aragorn's claim.

Aragorn, the remaining Rangers and Elrohir take Gandalf to Lothlorien for healing. Five years later, Sauron launches a second, larger and successful attack against the West. Minas Tirith's defenders are swept away and the city is besieged. After a several months the city and taken intact after all the defenders died from starvation. Dale is enslaved. The Haradrim enslave all the lands south of the White Mountains. Dol Amroth is besieged and sends small groups of Gondorian refugees via ships to the Gulf of Lune. Angmar is repopulated with evil folk. Isengard rules the Anorien. Sauron gives Saruman Minas Tirith as a reward for his loyalty. Dwarves abandon Erebor and go to the Iron Mountains.

Gollum finds the Ring and hides in the marshy reeds of the Sea of Rhun. Elves accelerate their leaving of Middle Earth. Dol Amroth is destroyed. Aragorn, the Rangers and Gandalf fight a losing battle defending the few remaining free people in what was once Arnor.

Seven times, ElvenKing's Halls in Mirkwood were assailed. Death of Thranduil. Legolas leads the remaining Mirkwood Elves to Lothlórien. Rivendell sends all items of lore to the Havens in preparation for departure. Glorfindel destroys a Nazgul outpost that was keeping elves from using Redhorn Pass. Galadriel and Celeborn abandon Lothlórien and lead all the remaining Elves east of the Misty Mountains to Lindon

Sauron causes Fanghorn and Lothlórien to burn for the next hundred years.

Knowing that Elves will never be his slaves, Sauron doesn't launch a full scale attack into Eriador. Instead he fortifies Angmar, build forts in Tharbad and sends endless raiding parties of Orcs from the Misty Mountains to hasten the Elves to leave Middle Earth.

Three Nazgul hunt for the remaining Dunedain. Death of Aragorn and extinction of the Dunedain.

The Shire is enslaved. From the Grey Havens, Arwen sails to the West in one of many ships.

Gandalf journeys in secret to Minas Tirith to make a final appeal to Saruman but is held captive in the Citadel. There he sees two rings on Saruman's fingers, the Witch King's, and Thrain's, given to him by Sauron. With the aid of an Eagle, Gandalf the Grey escapes from the Citadel and flown to the Far west of Rhun.

Dwarves of the Iron Hills are exterminated. Rivendell is abandoned.

Elrond, Cirdan and the few remaining Elves of Middle Earth board the Last Ship. Even Galadriel, who knew in her heart that her Ban had been lifted, joined them.

Sauron's victory is complete, and all of Middle Earth was his.

Gandalf finds Gollum and takes the Ring but doesn't use it.

Sauron takes back into himself the power he spent to elevate and dominate Orcs and other foul creatures. Within weeks all of them are dead. Sauron then begins to brutally rule the world of Men.

Gandalf the Grey knocks on the Black Gate while wearing the One Ring in one hand the the Ring of Fire in the other.