Fair enough. Can you tell me why?

If Luke dies in this movie, that would be lame. Like... are they going to just kill a legendary character per movie? That would be redundant and diminish the effect.

I think the title only refers to Luke and that's it. It'll be about his return. He's referred to as "the last Jedi" in episode VII, so I'm going to believe that's who the title is referring to. Rei isn't going to be a Jedi just yet. She'll go through training first. IF luke wants to take the risk of having an other padawan again...

OK, well I see it like this.

Let's take the three main characters from each movie.

Rey - We see her living alone in the wilderness, living a life of solitude. She's lying to herself about her family coming back. She's naturally gifted with mechanics and language. She discovers she has powers that frighten her. We see the universe open up in front of her ("I didn't know there was so much green in the whole galaxy"). We see her experience joy and fear and anger, and we see her chase her destiny.

Finn - We see a soldier born into combat who refuses to fight. A man who's willing to lie and betray his fellow soliders to save himself. We see his inner conflict: will he leave the Resistance or will he stay and fight. We see his loyalty to and perhaps his romantic interest for Rey. Totally outmatched at the end of the movie, he picks up a lightsaber and tries to fight Kylo Ren -- this is a powerful character moment.

Kylo Ren - We see a young man full of power and potential who can't control himself. He struggles with his family legacy, demonstrating strong feelings for both his father and his maternal grandfather. He's burdened by fear and quick to explosive fits of rage. He's being torn in two opposite directions.

OK, Rogue One...

Jyn - We see her as a child briefly and then immediately as an adult. She seems to carry some complicated feelings for her father, but those feelings are never really unpacked. She wants no part of the rebellion, no part of anything political,  but by the end of the movie she's giving pep talks to the entire Alliance. I still don't know what motivated her. Her transition from rogue wolf to rebel cheerleader is a function of plot, not emotion.

Cassian - We see a morally ambigious Rebellion spy who's not afraid to kill. There's a lot of potential there but it's squandered. He gives a soliloquy near the end about terrible things happening to him but that's all we get. 

Orson Krennic - He's a bad guy who feels like Tarkin is stealing his credit. I didn't get much else.

I'll agree K-2SO is a great supporting character. But should your droid/comic relief be your most engaging and dramatic character?

Rogue One treats its characters as props. They're just there to push from one planet to the next, from one action scene to the next.

QFT. The female Hayden Christensen.