If you're looking for an interesting take on the dark side, my opinion is to avoid the Sith entirely and look at Jedi Master
Rajivari and his followers. For those unfamiliar with this aspect of SWTOR lore, Rajivari was one of the original members of the first Jedi Council, centuries before the Republic was founded, and also became the first Jedi Librarian. At some point, he believed the Jedi had lost their way, and wanted to rebuild it from the ground-up to be more ruthless towards evil, and for the Jedi to rule outright. Essentially he wanted to start the Jedi over, "the right way this time." For those familiar with DC Comics, think of them as the Sinestro Corps to the mainline Jedi's Green Lanterns.
Rajivari and his followers were destroyed on Tython, but apparently he'd safeguarded
a huge collection of Jedi lore considered safe for Jedi consumption, and was able to stick around as a Force Ghost after death. What survives of the
Principles of Rajivari aren't dark in the Sith sense of being all about power for power's sake and the supremacy of the self, but dark in the sense that they advocate using ruthless measures to remain in power and mercilessly wipe out evil:
"Principle Two: All life is a battle, even to the last breath."
"Principle Five: Showing mercy to an enemy creates a spiral of destruction."
"Principle Seven: Sacrificing your strength is the path of a fool."
All interesting stuff—while flawed, it's still a lot more compelling than the equivalent of an addict's rationalizations for why they need their next hit of magical Emotion Meth.
Honestly, Ghost!Rajivari and his followers, returning from the Deep Core to retake the galaxy from Luke and Leia's "usurpers and false Jedi," might have been a way more compelling antagonist force for the Sequels than the First Order.