The Last Jedi various possibility is develop as star wars fan is base on various trailer provide various speculation how the coming movie story will go on. It interesting to see how the basis they draw their speculation.
Basically few main one I come across, Luke Skywalker turn to dark side, as Leia will died in this episode. The episode marketing material make Luke Skywalker had dual display on both Light and Dark side also provide a good hint for this.
It is interesting to see Luke Skywalker turn to dark side as the story go. This is why the tittle The Last Jedi.
Jey as the new form of Gray Jedi, this is why the last Jedi.
And ever speculation the Supreme Leader Snoke was once Jedi. Snoke is the leader of the First Order, a military junta resurrected from the fallen Galactic Empire, which seeks to reclaim control of the galaxy. Powerful with the Force, he has seduced Ben Solo to the dark side and become Kylo Ren.
Regardless of where the logic those speculation make the movie even eye catching whether or not it is fall under one of the fan propose and develop theory.
Base on the last episode can even saw the story Kylo Ren (Be Solo) kill his father Han Solo, I can expect this episode Kylo Ren (Ben Solo) kill Leia with the attempt to build up the story is nothing surprise. As the outcome for Leia, his brother Luke Skywalker turn to dark side of the force is possible, or maybe Jey turn to dark side is possible follow Kylo Ren.
As from the Skywalker family tree, we can see the Star Wars is actually all about Skywalker family, and Ben Solo is the chosen one left over. I will speculate Luke Skywalker can die in this two episode, and mostly Ben Solo will survive. Or maybe make the story turn from Kylo Ren back to Ben Solo. Else I can not imagine why need to make special life event for him to kill of his own parent (I assume now Leia will died as will on this coming episode). The logic behind is to build up the character will strong background and context to move the story forward for the last chapter for the Star Wars chapter 9 that come.
Off course the above speculation will be wrong, if Jey indeed daughter of Luke Skywalker is some how established, then the chosen one may be a Jey in return (the family tree expand as the story go). Base on current story development, I still think Jey is the one with the force, and she will be the last Jedi. As Ben Solo is somehow complete on his dark side training, who will want the person who kill both his parent be the super hero? ethically it impossible. Anyway, just speculation like the rest of people.