@andre:
I feel sort of bad for JJ. I can't think of a harder film for him to have to follow up on, not just because of fan rage, but because Rian left the characters in such weird places and didn't at all make the villains seem worthy of finishing the trilogy, thus Palpatine. Kylo just looked so pathetic at the end of TLJ, that I had no interest in seeing what would happen next. What could happen? We have ample proof that Rey is better than him at this stuff, and Hux seemed to be just as incompetent. Palpatine was not the salve that salvaged my attention at all.
I definitely agree that they should have planned a full story out ahead of time. Capitalism's impatience is its downfall. That being said, I don't think it would be impossible to salvage things for the third movie. Let's break it down:
1. Who are your protagonist and antagonist/deuteragonist? Rey and Kylo Ren.
2. What are their character arcs? Rey has no legacy and seeks one, Kylo Ren has a legacy (the Solos and Skywalkers) but seeks to break free of it.
3. Kylo is tormented by his bonds to the past, he seeks Rey to fill a void in himself.
4. Rey is tormented by her bonds to her lack of past, she seeks to fill a void in herself.
5. Rey and Kylo understand one another and want to fill the void in the others' hearts.
6. Rey's crusade should be to teach Kylo Ren that he can fill the void in his heart without violence and oppression.
7. Kylo Ren has to let go of the past.
So, how do we accomplish those things? Clearly, what we need is a dialogue between the two. This should be a segment of the film free of extraneous cutting-back-and-forth like Star Wars is so obsessed with doing. Put Rey and Kylo together on a backwater planet. Let them interact face-to-face and influence one another. Have Kylo help Rey save the indigenous people from some sort of natural event (dam breaking or hurricane, perhaps). Force Kylo Ren into situations where he is forced to confront that he can fill the void in his heart through positive actions. He can break free of the shadows of his family and be his own person.
That should easily be the focus of the first twenty-five or so minutes, give or take if you want to include an opening fighter scene that leads to the two crashing on the backwater planet. Positive feelings are like seeds, they thirst incessantly if not watered, therefore Kylo could then spend the rest of the film post-rescue consumed by conflict at his actions. "Yes, I can burn to sate my pain…but I can also protect to fill my void". Hux can then take over the First Order entirely and exile Kylo Ren for betraying them through his inaction and indecisiveness.