What if Luke Skywalker was a Sith Lord?

Fan made "Dark Luke" poster author unknown.

THE LAST JEDI is written and directed by Rian Johnson and produced by Kathleen Kennedy and Ram Bergman and executive produced by J.J. Abrams, Jason McGatlin, and Tom Karnowski. - StarWars.com

STAR WARS: THE LAST JEDI is scheduled for release December 15, 2017.

"John Boyega (Finn), Daisy Ridley (Rey), Oscar Isaac (Poe Dameron), Adam Driver (Kylo Ren), Lupita Nyong’o (Maz Kanata), Gwendoline Christie (Captain Phasma), Dohmnall Gleeson (General Hux), Mark Hamill (Luke Skywalker), the late Carrie Fisher (General Leia), Anthony Daniels (C-3PO), and Andy Serkis (Snoke) all return for the latest Star Wars. They’ll be joined by newcomers Benicio Del Toro, Laura Dern, and Kelly Marie Tran." - C

Who is The Last Jedi?

"This is the most obvious answer. The hero of the original Star Wars trilogy, and the guy whom the entire Star Wars universe seemed to be looking for in 2015’s The Force Awakens, Luke Skywalker is referred to as “the last Jedi” in the opening title crawl of the most recent film." -QZ

"Red is a significant color in Star Wars mythology. Sure, it was the identifier for Luke Skywalker’s X-Wing squadron in the original film, but the color’s overtones are typically insidious. It’s the color not only of many a Sith’s lightsaber, including Darth Vader’s and Darth Maul’s, but also the livery of the Imperial Guard from Return of the Jedi (arguably the most colorful costumes in the entire series to date). Even a long time ago in a galaxy far, far away, red tends to signify danger—at least for the good guys." -W

Dark Luke mashup by @Darkflame

"Peace is a lie, there is only passion.
Through passion, I gain strength.
Through strength, I gain power.
Through power, I gain victory.
Through victory, my chains are broken.
The Force shall free me."

There have already been theories that Rey might turn to the dark side during her training,

whereas Kylo Ren feels a call to the light.

We will have to wait! As wise Master Yoda says;