Here was Jason’s post about the MacGuffin.
https://makingstarwars.net/2019/02/what-is-star-wars-episode-ixs-macguffin/
It is described by his sources as being “delightful, but somewhat controversial”
If that’s true, that makes me think it will be something fans might already be familiar with. And it is something that could have big implications on the story.
Having it be the Journal of the Whills would be a deep cut into Star Wars history. We also know that George’s Original ST treatments may have something to do regarding the Whills, so it would be a cool way to tie the story back into his original ideas. And knowing George, their relevance probably would’ve been controversial.
https://nerdist.com/article/george-lucas-star-wars-sequel-trilogy-whills/
Also, since TLJ ended with Rey having some light reading material, it is possible she has learned about its existence and will want to go scavenge it for her collection. And Rogue One established Jedha as a place closely related to the Whills, with Jedha City being built around a Temple of the Whills, and natives like Baze and Chirrut being Guardians of the Whills.
We might not actually encounter the Whills, that might be too Lucasy, but they could have a prescence in the film through the existence of the Journal itself.
Another rumor mentions a possible threat from the Beyond, whatever that means. Could it be possible that the Journal could contain some crucial information regarding this threat?
Whatever the MacGuffin is, it definitely seems Force-related if both Rey and Kylo Ren are after it. It’s also possible that the MacGuffin could be some sort of device, like a Holocron or something brand new. So it might not be the Journal of the Whills, but if it was it would be a nice “bookend” to the story.
EDIT: I’ve also read a theory that the MacGuffin could be Kylo Ren himself. Maybe he gets betrayed by the Knights of Ren and the First Order and goes on the run, and both the Resistance and the First Order are after him. I could see that happening too.