I think it's kind of a given now. TFA/TLJ both have a bit of setup for them. The vision in TFA and a couple of mentions (Master of the Knights of Ren, etc), in TLJ there's the mention of leaving with a handful of Luke's students, and then there's the whole Kylo taking over the First Order thing. I think that it makes it pretty easy for them to go, okay well now he has a reason to have the KoR around as his personal guards, etc. RJ also said he didn't want to use the KoR as the guards in the Throne Room scene because that would be wasting them, and wouldn't make sense for Kylo at the moment.
JJ also said after TFA came out that if he ever did another Star Wars movie, he'd love to do a KoR anthology film. So, I'd be more surprised if they don't show up at this point.
I think ghost Luke is a given, too. I hope he shows up to Kylo, though. Would be reminiscent of his ghost bugging Cade Skywalker in the Legacy comics.
