The Outlander season 8 trailer has certainly had a lot of people talking since it came out. And guess what, the creators of the trailer have got you to do exactly what they wanted you to.
It’s all about the ending to the trailer. Claire looks off camera with a shocked face as someone says “Mrs. Fraser” to her. This has opened up a world of theories, from it being her father Henry to it being Claire’s brother. This is all based on events that are currently happening in Outlander: Blood of My Blood, the prequel to the series.
While I’m not dismissing the theories completely, I think the way the trailer was created was to get people talking. After all, the show knows that it does better when people are theorizing and discussing things, no matter how wild some of the theories are. However, trailers are regularly manipulated and cut so particular scenes don’t match the voice, and that’s what I think has happened here.
It sounds like Lord John Grey in the Outlander season 8 trailer
If you listen to the voice carefully, it sounds like it’s Lord John Grey who says “Mrs. Fraser,” and this could make a lot of sense. Based on leaked images of the final season, we did see that Lord John Grey was back, and he looked a little worse for wear. It’s likely that the season is going to follow the “Go Tell the Bees That I Am Gone” storyline and quickly wrap that up, meaning we get a story of John being held captive and William turning to Jamie for help.
If it isn’t John, then it could be William saying it. It doesn’t sound like him, though. However, I won’t rule that out.
I still think it’s John, and we know that he is going to be in the eighth and final season. John made a note of calling her “Mrs. Fraser” in the Outlander season 7 finale, making it clear that he is happy for her that Jamie is alive and that she is no longer “Mistress Grey.”
I don’t think the “Mrs. Fraser” and Claire’s reaction of “It can’t be” are part of the same scene, though. Instead, I think Claire is looking off camera at something completely different that is more likely going to come up earlier in the final season.

It could be a time traveler
I will say that it could be a time traveler, though. Note that John tends to say "Mistress" instead of "Mrs." as that was the notion of the time. For someone to say "Mrs." it would suggest that it's someone from the future.
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This is possibly why people have started to suggest Henry or Claire's unknown brother, but I wouldn't say that. There is another potential time travel who we have already met.
In the books, we learn that Captain Richardson is a time traveler. He does some side switching throughout, and it's highly likely that he comes to the Fraser homestead to speak to someone he knows to be on the Rebel side. It's all going to depend on whether Outlander sticks to the book storyline to make him a time traveler, though.

Brianna and Roger are likely back in Outlander
Claire’s reaction is likely to do with Brianna and Roger. At the end of Outlander season 7, the two reunited, along with their children, and they needed to figure out where to go next. It’s clear that the 1980s isn’t as safe as they thought, and we know from “Go Tell the Bees That I Am Gone” that they head to the 1770s again.
Claire is shocked to see her daughter back. She never expected Brianna and co. to come back to the 18th century, and she would have had no idea of the adventure Roger and Buck have had in 1739. So, to her, seeing Brianna, Roger, and the children walking up the hill likely seems impossible.
It’s fun to theorize and speculate, but at this point, I don’t see Outlander fitting storylines from Outlander: Blood of My Blood into things. There’s just too much to fit in to wrap up the story in the first place.
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Outlander is available to stream on STARZ.