Star Wars: The Last Jedi - 10 Predictions For The Biggest Ever Twist
4. The Jedi Were Wrong All Along

The Theory
What if everything the Jedi have ever taught about bringing balance to the Force was fundamentally wrong, and their lack of fluidity in definition has brought the galaxy to the verge of permanent damage?
There's a pretty pertinent theory that The Last Jedi will introduce the idea of grey Jediism through Luke, and will play with the suggestion that the old Jedi Masters were wrong to force an extremism-encouraging binary split between Dark and Light.
This would explain why Luke seeks to end the Jedi Order, which is too much of a door to the Dark Side: he wants balance that means accepting that Dark and Light have to coexist in harmony. Ironically, he could have a big ally in that agenda in the shape of Snoke, whose agenda to turn Kylo Ren to the Dark Side was revealed by Pablo Hidalgo in the Visual Dictionary of the Force Awakens:
“The Supreme Leader believes Ren to be the ideal embodiment of the Force, a focal point of both light and dark side ability."
Their end-goals might be different, but their rejections of the old binary ways would be a nice parallel for the sequel to make.
Would It Be Shocking Enough?
Ending the Jedi Order and challenging everything that the Star Wars mythology is based upon as a lie is about as world-changing as it could ever get. It might not have the same immediate emotional pull of an "I am your father" type situation, but it would cast out some pretty major shockwaves.