In a fanfiction where the Sith Empire prevails in Star Wars Clone Wars, the galaxy would be a very dark place. The Sith values of power through domination would mean the subjugation of countless species. Jedi would be either wiped out or forced into hiding. The very fabric of the Force might be warped as the Sith use it for their evil ends, like enslaving planets through dark side magic. It could also lead to a new hierarchy in the galaxy, with Sith Lords ruling over sectors, and any form of democracy or freedom would be a distant memory.
Power is a very common theme. Sith are always seeking more power, whether it's through the Force, political means, or by eliminating rivals. Another theme is betrayal. In the Sith world, loyalty can be a very fragile thing, and characters often betray each other for personal gain.
The plot of Revenge of the Sith centers around the final stages of the Clone Wars and Anakin Skywalker's descent to the dark side. It's a complex and intense story with lots of action and drama.
I'd recommend 'Sith Apprentice: Legacy of the Dark Side'. It's a story that focuses on an apprentice who discovers an ancient Sith artifact and the chaos that ensues. It's full of mystery and action.
One possible fanfiction could be about a Sith Lord from Star Wars ending up in the Naruto universe. Maybe he tries to use the Force to control the chakra, but finds that it's a completely different power system. He could then form an alliance with some of the villains in Naruto, like Orochimaru, who are also interested in power and dark arts.
One memorable OC Sith could be Darth Zyra. She has a very distinct look with her purple - hued lightsaber and her elaborate black and red robes. Her power lies in her ability to manipulate the minds of her enemies, which she uses to great effect during battles in the Clone Wars.
If the Sith allied with a villain in the MCU, it would completely change the power dynamics. Let's say they teamed up with Ultron. Ultron's technological might combined with the Sith's Force abilities would make for a formidable force. The Sith could use their mind - control powers on the Avengers, while Ultron could launch his mechanical attacks. This alliance would force the heroes to think outside the box and find new ways to combat such a strange and powerful combination of enemies.
The concept of the 'rule of two' is also frequently explored. In many Star Wars Sith reader fanfics, the reader has to deal with the implications of this rule, either trying to break it, uphold it, or find a way around it. It adds a layer of complexity to the relationships within the Sith order in these stories.