I liked a theory I read earlier - when the Jedi allowed "the will of the force" to determine what happened, when they were just "agents" of the force - there was no need for the "dark side" to rise for balance.
It was only once the Jedi hubris came to the fore, once they started manipulating the force to their will, that 'the chosen one' was created. Then, having successfully destroyed the jedi, his final act was to destroy the evil that was now out-of-balance.
Luke, as uber-powerful as he was, was an observer of the force - he knew that by bending it to his will, he would only create a counter-balancing dark force user. By closing himself off, he removed/reduced the force abilities of all force users - perhaps this is why Kylo is so poor.
This led me to the following retro-fitted theory:
The "awakening" is not Rey, rather a dark side user from uncharted territory - Luke's sacrifice on Salt-planet was not just to aid the rebelli...resistance escape, but to move his consciousness to an elevated plane where he can influence the cosmic force to counter this new rising evil.
It's why Rey has developed without limit nor reason - with Luke shut off from the force, she has been chosen to embody the force. Why she can match/defeat Kylo (she has god mode enabled), and why Luke believes he had to die (his very existence diluting the force power in Rey).
And now for the 'amazing' (cliche) twist ending....Power corrupts, and absolute power, corrupts absolutely.
Rey will be seemingly defeated by this new evil, Kylo defends her at the last minute and combined with Astral Luke they will defeat/destroy the ultimate enemy. Luke's journey now complete, he has his heartfelt goodbye, fading (ala Yoda) with Rey and Kylo facing him, only for the final shot of the film to be from his point of view, Kylo with Rey offset behind, fading to darkness. As the screen goes dark, the sound of a lightsaber igniting, then thrust towards the camera - highlighting Kylo's face as it punctures through his chest from behind.....The Emperor cackles.