I know this is going to be blasphemy for some, but the Outlander books aren't perfect. There are forgotten characters now and then, and one of the worst is forgotten in Go Tell the Bees That I Am Gone. It's all about Buck MacKenzie, who is presumably left behind in 1739 in the books.
In the TV series, we've come to know Buck MacKenzie as a reliable and heartfelt man, despite the initial introduction. He joined Roger in the hunt for Jemmy in the past, and he even went on the wild goose chase that ended with Jerry MacKenzie being found and sent back to his own time. We've come to know Buck well throughout Outlander season 7, leading to some positive feelings for the character, and that means Outlander season 8 needs to change things from the books.

Buck MacKenzie can't be left in the past in Outlander season 8
One of the issues with Buck's story is that we know he "dies" in 1778. This could mean that he doesn't make it back in 1778, but it could also mean that he actually makes it back and dies. It's a mystery that Outlander season 8 needs to answer, because Go Tell the Bees That I Am Gone doesn't.
In the ninth book, Buck isn't even mentioned when Roger and Brianna return to Claire and Jamie. The last we saw was actually a daydream of Roger's in 1739. He imagines Buck going into Geillis's house, sleeping with her, and then disappearing in a flash of blue light. Otherwise, we just know that Buck is physically attracted to his own mom, and that it looks like he's considering stying in 1739 rather than going back to 1778 or even back to 1980.
It's disappointing that we're just left with a forgotten character in the books, and the TV show can't do that. Every character we've grown attached to needs some sort of closure.

How can Outlander season 8 manage Buck's story
There is a great way to make Buck's story fit the narrative already told. While he could certainly decide to stay in 1739, that's unlikely to happen. With Roger, Brianna, and the children heading to 1778 — at least, assuming the show will follow the books — then Buck is likely to want to go with them. That gets him back to his own time.
When Buck traveled through the stones to land in 1739, he suffered from chest pains. The series didn't focus on them all that much, but it could be a sign that someting happens as he travels through the stones again, leading to his death in 1778.
Alternatively, he could get back to his family, agreeing to make amends when he returns. This could lead to him eventually dying in the same year anyway, his heart struggling from the weight the time travel trips have put on him.
Whatever the show does, I need to see Buck again. I, at the very least, need to see him mentioned in Outlander season 8.
Outlander is available to stream on STARZ.
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