TLJ and the whole ST rests on this.
Is the ST a story based on Luke being a grand wizard on the "good" side, vying against a grand wizard on the "bad" side in Snoke, and both their apprentices (Kylo and Rey), caught in the melee, learning to hate each other and taking their fight thru the three movies? Where the good side beats out the bad in the end?
Is it only just that simple a story? Taking up that same old arc of the OT/PT?
Or, is the ST a story based on Luke and Snoke both being grand wizards who are both manipulating things in the galaxy for their own ends, or perhaps in cohort, and Kylo and Rey are therefore thrust into the picture more, realizing eventually they're both being manipulated, having to come together to stave off the wizards?
For the ST to break some ground, which by all reports TLJ does, then TLJ has to, being the middle of the trilogy, involve convoluted and complicated relationships between those main four characters (kylo, rey, luke, snoke). Machinations and double-crossing going on, mixed relationships. Kylo and Luke, Snoke and Rey, Luke and Snoke, Kylo and Rey.
I think TLJ, and the ST as a whole, is taking a turn where good and evil are being blurred, where these main new characters (Rey/Kylo) are at the forefront of the story, whether in romance or as siblings/cousins, having been manipulated by warriors/wizards of the olden times and the old ways, or realizing their old ways are no longer viable, and needing to pave a new way forward because the galaxy had fallen into a history repeats, the constant Sith vs Jedi philosophical differences being the center of turmoil in the galaxy for thousands of years.
So, such a more complex story will mean one or two much-loved OT characters (namely Luke) has to be re-envisioned, has to either fall to the dark side, or shown to be in league with Snoke. Something of that order. But not the same refrain of Luke vs Snoke as the gist of the ST, like Yoda vs Sidious in the OT/PT.
It can't be a simple good vs evil story, where there's a larger known "good" and a larger known "bad" character driving things. Instead, and especially for the fans, a story told where lines get blurred, previously liked characters are made a little more unlikeable, and previously disliked characters are made a little more likable, where hidden agendas are exposed, failings, more humanity, compassion and understanding going both ways, like Adam Driver said of TFA where Kylo is a villain but not from his perspective.
Bottom line, many fans I envision are going to be a little upset that the fantasy world of SW's gets a little more real, where certain OT characters lose some sheen....like Han and Leia broken marriage, failed raising a kid, Han a bad father, Luke a broken old man and/or with dark side machinations, and perhaps Leia too will be shown to have some poor characteristic in TLJ. Not destroying the OT characters, but certainly re-envisioning them all from the happy go lucky heroic and all "good" personalities we fell in love with them for in the fantastical world of the OT.