I stared earlier this year that I feel the theme of "prophecy" will play a factor in the next film, maybe going as far as having it in the actual title. I believe Rey and Luke's training will coincide back and forth with Kylo and Snoke's.
With that, I also believe, from the tiny hints in the Art of RO book, that Vader's castle will be used again (and what better time to use it when it was just shown in a previous film and will be fresh in everyone's minds) and Snoke will show Kylo the Sith cave underneath the castle. Back on Ach-To, we'll have Luke show Rey the first Jedi Temple (which I assume is part of that tree we've seen in set photos).
Piling this all together, I think we'll get more background on the Whills, from both parties. Perhaps, Snoke's and Luke's story about them are totally different, leaving the audience guessing which one is correct (in which we'll discover in EP 9?). Or maybe Luke has a piece of the Journal of the Whills and Snoke has the other half and in order to fully understand the prophecy or Force, they need to connect the two?
Regardless, I believe the next film could be called Prophecy of the Whills. Or even Journal of the Whills.
(sorry for such a long post haha).