This alternative storyline strays far from Canon, so if you do not desire such, turn away now.
As return of the Sith is an major event in galactic history, time is now count by it. As a community member pointed out, Battle of Yavin does not happen so counting time by it isnt a way to go. Thus being years in timeline are marked with ROTS, as in Return of the Sith, and as BTROTS, for Before the Return of the Sith.
-BTROTS
The Galaxy burns. Clone Wars rage on for the second year. Neither the Galactic Republic or Confederacy of Independent Systems has been able to crush one another. Clones have proven far superior fighters, capable of overcoming the vast hordes of battledroids fielded by the Confederacy.
But their hordes seem endless. The war goes on, victory seems to be only a distant dream for either faction. Hutt space has remained out of the war, attempting to profit from both sides equally.
this was all coming to an end, for another force was about to enter the Confederate space. A force that would change the whole conflict permanenetly and rewrite galactic history.
-Late 1 ROTS
A scouting ship enters the Confederate space. No one knew what this would bring. Confederacy is quick to track it down and force it to surrender by superior numbers. Captain of invading ship requests audience with the leaders of the faction. Such is granted and a holocall between Count Dooku and the captain is established.
Soon it is clear who the outsiders are. Not an culture formerly unknown to the Galaxy. They're Sith.
For a thousand years they have awaited beyond the borders of the known galaxy. learning from the mistakes of the past they've found a way to co-exist, unified by a single motivation. Revenge. Preparing, watching the Republic weaken. Scouts were sent periodically to see in which situation the galaxy is. now had come the time to strike. A vast fleet of Sith warships enters the Galaxy, ready to conquer it and destroy anything in their path. But they had not prepared to face an enemy capable of fighting back effectively.
Count Dooku is quick to exploit this. He orders the execution of Separatist council members after they're brought before him under the guise of negotiating with the outsiders. Many Confederate worlds soon find about of what occured. Their newly elected leaders refuse to stay in the Confederacy and the faction is divided to fortress worlds loyal to the Separatist cause and worlds supporting the newly established New Sith Empire.
Dooku realizes his chance, he reveals Palpatines betrayal to both Jedi and the republic senate. The Jedi act swiftly, and manage to cut the still shocked Sithlord down during an failed escape attempt. The Galactic Republic is now leaderless. In the state of chaos, Republic manages to negotiate a temporary truce with the Confederate remnants, however the tensions with the 2 factions remain. Republic pulls its forces from battlefields all across the stars, as does Confederacy
-Early 2 ROTS
Meanwhile the combined force of Sith military and forces loyal to Dooku crush the Separatist worlds one by one, enforcing them back under the command of Dooku. The Sith admire the cunning and newfound brutality of the former Jedi-master. It is unknown how it exactly occured but soon dooku has become the new Emperor Tyranus. Within Months, New Sith Empire has enforced all the rebelling worlds into its ranks, and named puppet lords to lead them. Despite these campaigns being costly, they're now ready to face their eternal enemy.
Time of infighting is over. Now is time for war that would bring the Galaxy under the whim of the Sith once and for all.
But the Republic has not wasted time preparing for what is to come. Facing the realization that an old enemy has resurfaced mightier than ever and absorbed the Confederacy into itself, A new Chancellor is elected, with the choice of the people being Wilhuff Tarkin, who rapidly militarizes the Republic in preparation for inevitable war. The jedi reveal information about who this enemy is and soon the long forgotten stories of their brutality and destruction they bring have spread everywhere in the Republic. Facing the possibility of total annihilation, republic officials put forced conscription into use, recruiting vast amounts of human civilians into the Grand army of the republic. The Jedi realize that they're about to face an foe with vast numbers of force-sensitive warriors in their ranks. Higher focus is put to combat training and many more Jedi are deployed to serve as commanders of the army, not just as generals but now also as squad leaders. By the time the New Sith Empire is ready to strike, many republic worlds have been heavily reinforced and fortified to withstand the waves of attackers.
-2 ROTS
The assault of the Sith is launched against numerous worlds simultaneously. believing that the Republic can not survive this fight, Emperor Dooku uses large amounts of his forces in these attacks. Overconfidence costs him dearly.
The Sith have suffered many casulties fighting against Confederate remnants and now their numbers are too few to overwhelm the Republic as Dooku expected. Battles rage on for months, with neither side gaining or losing significant ground. Eventually heavy fighting wears out both factions. The Sith withdraw their remaining forces and start fortifying their own worlds. Unable to break one another, the bitter fighting continues throughout the Galaxy in smaller conflicts, leading to little gain or loss. The war forms into an stalemate.
With no end in sight, Every major power across the stars prepares for years of war to come. Whispers of the chosen one are heard once again. Will an agent of light, or darkness come forth to destroy the Sith and bring balance to the force, or leave it in darkness in the ruins of the jedi temple?
The story was edited on 15.4.2021, a few things corrected.
I think its a pretty cool story, especially with tarkin becoming chancellor.