Daisy plays Kira, a character living on Tatooine with Max Von Sydow's character. Kira is a dusty survivalist tomboy. Sydow is a broken man from the prequel area that has become mostly cyborg. He is not force sensitive and for some reason collects artifacts that we've seen at critical points during the previous films, particularly ones relating to the Jedi and the Sith.

The "fact" that this character is not force sensitive probably means it's not Anakin or any other former Jedi / Sith... probably Boba

Anakin Skywalker.

Evidence:

He's a prequel character.

Sith connection.

His identity will shock the audience. Is some second-tier character going to shock anyone?

He's living on Tattooine, his old home. Why would the movie go back to Tattooine again? The Skywalker family is the only connection to that world.

He's a cyborg.

He's watching over his granddaughter.

Max von Sydow is in his eighties, the same age Anakin would be at this point in the timeline.

There's a Darth Vader flashback that takes place on Endor. In this flashback we will learn that Darth Vader/Anakin Skywalker survived. If they're going to do a flashback with Vader, it must be for something huge. This is Luke's stunning reveal.

It might even explain why Anakin's 'force ghost' in the last ROTJ revision was that of his younger, pre-Sith self. Luke never actually interacted with the ghosts at the end, so they might have just been a vision of some kind. Also keep in mind that we never saw Anakin/Vader fade away to become a force ghost, and he was never taught how to become one, either. This also mitigates a sucky change made to ROTJ if those weren't actual force ghosts Luke was looking at.