What if the Sith Empire still existed?

After the Great Galactic War, the known galaxy was split in half between the Galactic Republic and the Sith Empire. And eleven years later, a war between the two galactic superpowers was reignited. However, the war was interrupted by the sudden arrival of the Eternal Empire, which was ruled by Emperor Valkorion, who is in fact none other but Darth Vitiate, the Emperor of the Sith Empire, himself.

But eventually, the Eternal Empire was defeated by the alliance between the Empire and the Republic. This also led to the third war between both sides as well. And as many centuries passed by, the Galactic Republic begins to recover the territories lost to the Sith Empire back until it has become sole dominant galactic superpower in the galaxy.

So what can be done to stop this? What can be done to restore the Sith Empire to its greatness and put the Galactic Republic on the defensive instead? And who among the Sith in this age should be the one to do it?