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.
The rise of the Sith Empire in Clone Wars fanfiction could start with a series of strategic moves. The Sith might have infiltrated key positions within the Republic long before the Clone Wars even began. They could have manipulated the creation of the clone army, for example, ensuring that they had a way to create chaos and further their own goals. Once the war started, they could have gradually turned the tides in their favor by making secret alliances with certain separatist groups and using their dark side powers to influence key battles.
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.
One way to create an OC Sith Lord in Star Wars The Clone Wars fanfiction is to first design their backstory. Maybe they were a former Jedi who was seduced by the dark side due to a personal tragedy. For example, they lost their master in a cruel way and blamed the Jedi Order for it. Their appearance could be menacing, with dark robes and a red - glowing lightsaber.
A popular one is 'The Sith Apprentice's Trial'. In this fanfiction, the apprentice has to face numerous challenges to prove their worth in the Sith order. It has some really cool battle scenes and explores the darker aspects of the Star Wars universe in an interesting way.
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.