This also brings up the question of who can even see Force ghosts?
Leia apparently couldn’t see them at the end of ROTJ, so is it a matter of one’s own ability with the Force? Maybe Force ghosts can choose who they appear to? Maybe it is a bit of both, since Yoda couldn’t appear to Luke until he reconnected with the Force.
To build onto that, I personally like the idea that Force ghosts can only appear to people they knew while they were alive. This might seem limiting, but I think there needs to be some kind of limit, because without one it kind of lowers the stakes of Luke’s importance. For example, even if Luke didn’t pass on what he learned, Yoda or Obi-Wan could just appear to anyone who was strong enough with the Force and teach them how to be a Jedi even after Luke was dead and gone. But, if Luke dies without teaching anyone, then that knowledge will be lost forever.
But now that Rey has met Luke, Luke will be able to come to her as a Force ghost and continue her training in IX, presumably.
This idea would also explain why Luke didn’t see Qui-Gon at the end of ROTJ. A Certain Point of View showed us that Qui-Gon had learned to manifest himself by the time of ANH, but despite that we don’t see him at the end of ROTJ.
To continue, if you carried this idea forward, that would also mean that Kylo Ren/Ben Solo couldn’t see Anakin since he never met him. But, it would kind of answer why Anakin never appeared to him before to stop him from going down that path in the first place. And maybe someone’s depth within the dark side almost makes contact difficult or impossible. This would explain why Obi-Wan or Yoda couldn’t help Luke fight Vader and Palpatine. Obi says in ESB that if Luke faces Vader that he can’t interfere. Yes, you could argue that there are “certain rules” he can’t break, but I just personally like the idea that dark siders can create “interference” that make it difficult for Force ghosts to manifest near them. This would help explain why Force ghosts can really appear to give advice and not really help in a battle with dark siders.
Also, maybe Force ghosts are both an independent manifestation of the Force, but they also can only manifest physically by using a living Force-sensitive individual as a conduit. Yoda not being able to appear to Luke until he reconnected would explain that. If Yoda was solely an independent manifestation, Luke’s own connection to the Force wouldn’t have mattered. And clearly it comes down to the individual, because Rey, another strong Force-user, her presence didn’t allow Yoda to manifest in front of Luke, either. And Yoda never appeared for Rey to give her advice on how to get through to Luke. Sure, you can say that Yoda wanted her to figure it out on her own, but I think physical limits to her abilities would allow you not have to to resort to handwavey explanations.
Maybe in extreme situations the rules can be bent, like in visions or dreams. Sort of like how Rey hears whispers of Obi-Wan during the Force vision. So maybe Kylo could still see Anakin/Vader in a dream or vision, but not in a regular old conversation.
A Force-user’s perception could also possibly effect how the Force ghost is manifested. So Force ghosts could either choose how they want to appear, or maybe the conduit’s own imagination or memory of an individual could effect how they appear to them. If Luke had saw old holograms of young Anakin Skywalker, it could also explain why Luke perceived him that way in ROTJ. But the canon seems to imply their appearance is the ghost’s own choice, so maybe Anakin just wanted to appear young.
I think the writers will do whatever is necessary for the story, but I do think Force ghosts need some kind of limitations in-universe. Otherwise, they become a Deus Ex Machina that raises questions of why they didn’t just do this or that in the story.
We know Luke will appear as a Force ghost in IX, so maybe the extent of his abilities will broaden our view on Force ghosts and throw all my ideas out the window. This is just how I like to perceive it as of now.