Revised dialogue for opening sequence (edited again August 5):

OVERSEER: At last, Snoke’s apprentice graces us.
KYLO: I killed Snoke. I’ll kill you.
OVERSEER: Such hate! More than enough to hear the wayfinder’s call. And it led you here. Just as he knew it would lead someone worthy.
KYLO: Who?

Kylo finds the cultists working at cloning tanks, we see new shot of Sith troopers assembling, maybe a holographic memorial or statue of Palpatine. Ideally the clones would have Jango’s face. At last Kylo reaches the Overseer.

KYLO: Who are you? Sith?
OVERSEER: No. We are but worshippers of the Two. For a millennia we served them. Even in their deaths we remain faithful, for the dark side is eternal.
KYLO: What is this?
OVERSEER: While Brother Snoke prepared the galaxy for a return to order, we kept watch over the jewel of the Emperor’s Contingency. What was once Snoke’s inheritance is now yours to claim.
KYLO: What could you give me?

[cue Star Destroyers rising to the sky]

OVERSEER [partly voiceover during fleet footage]: All that remains of the Imperial Starfleet. Hidden away years ago, left to our care…the final order of Lord Sidious.

This provides a succinct explanation of who and what we’re seeing, makes clear the Overseer himself isn’t significant, establishes their ties to the Sith without the Sith themselves returning, adds a bit of lore to Sith history between Bane and Sidious, suggests a simpler explanation for who Snoke was, ties the plot into/pays off the Aftermath/Shattered Empire/Battlefront II setup about the Contingency, further preps the audience to recognize Rey’s ability to sense the dagger’s connection to the wayfinder as a dangerous sign, and strengthens the final chapter’s sense of finality, by more clearly suggesting that once this fleet is finished the Empire will truly be gone.