Sith Troopers, Project Skywalker, Knights of RE(va)N, and the Kyber Crystals
Buckle up, this is going to be a long bumpy ride with quite possible Star Wars 9: the Rise of Skywalker spoilers if I'm right. Ye be warned.
Things that've been confirmed for tRoS: Palpatine, Death Star Wreckage, and Sith Troopers. And based on all that, here's where I think we're going.
Sith Troopers are a strange name for First Order troopers when we know neither Snoke nor Kylo were considered "Sith". We also know Palpatine survived (or is at least in tRoS in some fashion), but given how the line "No one is ever really gone" was followed by the unmistakable cackle. I think we can safely assume that there is a physical presence (or dark side possession of someone) by Darth Sideous. We know full well he was always searching for ways to extend life beyond the normal.
So these specialized troopers are likely assigned directly to good ol' Plaps who's hiding in the shadows, playing the galaxy like he always has. (they might even be Revan's carbonite frozen troops!) The question is who he is possessing. Is it a clone like he did in Dark Empire (ala Matt Smith) or are they leaning on the new canon (ala Lord Momin) and his Sith spirit is tied to some object?
Regardless, the Sith Trooper seems to signal a physical presence vs perhaps a hologram as some have speculated. But what has ol' Papls been up to this entire time?
That's where we get to Project Skywalker. As we found out in Rogue One, the Emperor loves his project code names. We had Project Stardust, Project Darksaber, etc. So why not Project Skywalker? Theoretically, Project Skywalker started as a joint Sideous/Plagueis effort. As the Sith Lord could "even use Midichlorians to create life." I think this is even where the title of the film: the Rise of Skywalker comes from.
While a recent comic seems to imply that Palps was the creator of Anakin, the movies hint heavily that it was actually Plagueis. Why would Palps create Anakin and just abandon him? Far more likely this dream is just that and Anakin projects his fears. More likely still, Plagueis had Project Skywalker started, didn't tell his student where the boy was and when Sideous turned on him, the boy was lost track of. However, once Anakin was introduced to Palps, he recognized the boy but could do nothing as he was in the care of the Jedi. The events of the Clone Wars unfold and Project Skywalker continues (and we continue to see it in Rebels) as the Sith continuously kidnap force-sensitive children as Palps attempts to reconstruct his masters work.
Now we get to where Rey comes from. She is a part of Project Skywalker (and she rises!) the continuous effort of Palps to create the perfect force user (perhaps as a host). Except she's abducted from the program and lost or abandoned on Jakku to fend for herself. Hence all that power. Maybe she's grafted from cells directly from the Skywalker line (Palps would have had easy access to them), or maybe she's just infused like Plagueis claimed to do.
But now she's up against Ben/Ren and the Knights thereof. So if the Knights of Ren predate Kylo Ren, where is everyone getting this name from? Revan. They are an order based off Sith principles and it's not hard to imagine that a 4000 year old data file describing Revan was so damaged that his name was distorted into Ren (or perhaps shortened to hide the truth). As a powerful Sith lord that even Palps studied, Revan had many followings. It only makes sense and Kylo's mask feels like a tribute to the Sith Lord of old.
With Disney's next trilogy supposedly focusing on the Old Republic and very potentially that fan favorite time frame it only makes sense for them to plant the seeds now. And they seem to have resurrected the Sith Trooper (who were first seen in Revan's service.) But why aren't the Knights of Ren (some of whom may be fallen students from Luke's academy) armed with lightsabers? Well you can thank the Empire for that.
As we all know, during the time of Rogue One (and referenced in the Clone Wars), the CIS and the Empire made great effort to harvest all of they Kyber crystals they could get their hands on for Sideous' super weapon, the Death Star. They snatched up every crystal they could find and destroyed many more sources, rather than let them fall into enemy hands. No kyber, no lightabers.
So when you are Rey (or the Knights) and you need kyber crystals, where do you go to get some? The wreckage of the Death Star perhaps. It's been rumored that Poe may be force sensitive (in the new comics) and if Rey needs to train him to aid in the fight against the Knights or Palps... where else is she going to get some?