We'll find out that Snoke, Kylo and the Knights of Ren wanted to keep knowledge of the Force, to themselves, partly to keep the chaotic galaxy under control, partly to just hoard it like the 1%. They feared that the awakening of the Force, and teaching it to so many would lead to "un-deservings" have access to a power that would threaten their own position in the galaxy.

We'll be introduced to the Knights of Ren, and we'll find out they're far more sadistic than Kylo.
Kylo will probably end up being critically injured or killed trying to defend Rey from the Knights of Ren, but she'll end up finishing the job herself. Kylo will lament his path via dying last words to Rey.
The resistance will win, but we'll find out there's something else behind Snoke/The Knights of Ren/The Darkside of the Force, and we'll see Rey and company freeing the child slaves from that Las Vegas planet, and it's going to be implied that not only will these kids be the next generation of Jedi, but many others have the Force ability as well. Maybe Finn will display some kind of connection to the Force in this film, that might lead to the possibility of him being the lead in Rian's upcoming trilogy.

I can't see Kylo saving Rey. It's not a good look. He'll probably turn on his own knights when it looks like they'll kill her, but she'll be the one to actually succeed in killing the "Jedi Killers." They don't want to revert back to the damsel-in-distress cliche.

As far as the kids go, it seems weird that Finn and Rose left those kids as slaves, while freeing the space horses, so that's why I think they'll return to it. Plus, I think they want to make the Force more inclusive, power-to-the-people and all that, so that might be a way to make that connection.

I like the idea that Kylo turns on the Knights, not necessarily to save Rey, but to turn her at the expense of his Knights. It fits more with his conflicted state. Instead of doing a redux of Darth Vader redeemed, Kylo saves her but for selfish reasons. It can also tie into what was established in the prequels, that the Sith always betray one another.

I don't see why Rey killing the Knights of Ren would make her look like a villain. I like to think that as enlightened as the Jedi are, that they would have a Superman-like no killing policy, but they wield glowing samurai swords. Their business is to separate heads from necks. Plus, Rey killing them would probably be in self-defense. Her and Kylo already killed the Crimson Guard.
Off topic, but I hate how they portray some of the killings in these movies. Rey kills half of Snoke's guards, all of them faceless and voiceless, one them even going comically into a paper shredder. No sadness or remorse about taking a life. And later when she's shooting down First Order pilots in the middle of a bloody battle, she's geeking out, all happy, "I like this!!!" as if she's playing Call of Duty.

Rey having to be saved...I disagree. I agree that Rey having vulnerabilities or losing at times doesn't make her any less of a "bad ass." But on the other side of the coin, the original Star Wars had a habit of subverting expectations. Most films the hero does the right thing, slays the dragon, saves the day, and the opposite happens in TESB. Star Wars tries to turn some cliches on their heads. The damsel in distress is already a cliche, so that's why I don't think they'll do it.

The kids from TLJ, yeah, it took me a little bit out the movie. Like those commercials from the 80s where they had kids playing with Star Wars/Transformers/GI Joes. The ring looked like something that would be included with the DROIDS cartoon action figures. Also, it reminded me of the Jedi younglings.
But that's an excellent point about everyone being Force sensitive and Rey's power levels. It's implied in ESB that it takes time and patience to learn the ways of the Force. But someone like Vader took the shortcut to get these awesome powers and it ended up corrupting him. So if someone like Rey is already super powerful, enough that Luke's afraid of her, what's so tempting about taking the path of the dark side?

NOT have Ren get redeemed? His loss in the first movie, him getting outsmarted in the 2nd ... all this combined with his already fragile emotional state, just pushes him completely into insane territory where all he wants is power. So he gathers up his Knights and they just go on a galaxy wide spree of destruction