Ok so this is something that I've been speculating on every since the lines "I Will Finish What You Started" and "Im Immune To The Light" were leaked through those Star Wars themed make up accessories (weird way to find be exposed to such important dialoge, I know...)

First of all I'm going to make alot of assumptions regarding the characters origins so bare with me.

So basically I believe that Kylo is the son of Han and Leia while Rey is Luke's daughter and that (just as in real life) Adams character (Kylo) is is roughly 7-9 years older than Daisy's (Rey) and that at at a young age roughly 10 years after ROTJ, Kylo was taken by the KOR (thought dead by Han and Leia, which prompted Han to take off and leave Leia) During the attack however Luke managed to escape with an infant Rey and leave her on Jakku, to be watched over by MVS's character maybe?

Kylo (who again, was not as lucky) was indoctrinated by Snoke and his followers into believing (like all FO loyalists) that Darth Vader was a great leader who basically helped establish and expand the glorious and powerful imperial empire. He was told that Darth Vader was also responsible for wiping out the corrupt and "evil" (from Kylos POV) Jedi Order...

Now I think that the force has been dormant (to some degree) in the years leading up to TFA and that the only person strong enough in the force (the light side specifically) to actually be able to communicate with the Jedi of Old (force ghosts) is Luke (if anyone......for all we know it's been quiet on Luke's end for a while now).

I think the reason why Kylo says he's "immune to the light" is not just because he's being a bit of a darkside snob haha but also because maybe training in the force (with Luke when Kylo was young) was a struggle for him. And now Kylo is convinced that it was because he was always destined to walk the path of the darkside and continue his Grandfather's work.

I don't think force ghost can just pop up in front of whoever they want too....I believe that an individual has to be attuned to the lightside to some degree in order to communicate with the "other side". So basically Anakin might want to speak with his Grandson and try to keep him form following in his footsteps...its just that Kylo has fallen too far into the Darkside for Anakin to reach out and appear before him.....

Ok so this next part might make some people angry but please understand this is just what I believe to make sense within the context of the story...........I believe that for Anakin Skywalker NOT to appear at any point in time in the new trilogy would be very very strange and frankly would be a HUGE plot hole IMO.

This isn't about weather or not you like the PT or what your personal feelings might be towards Hayden Christensen .....what I'm talking about now is the character of Anakin Skywalker which hate it or love it....is Hayden.......and again...hate it or love it his force ghost is canon (check out Disney's Star Wars app if you don't believe me).

So with that said, while I never expected to here rumors of Hayden returning so early (episode 8) I can't say that I'm to surprised. Nothing is confirmed but I have a feeling that this is probably going to end up being true.....however I think people are a bit confused by the rumors. I think that that report of a UK team heading to the US to train Hayden is referring to Rouge one. I think that Hayden will wear the suit in Rouge one and will have a fight sequence in the movie near the end against the Rebels....

Five months later episode 8 is released (both film's are in production atm) and it is in this film that I believe Hayden will play a force ghost Anakin Skywalker in which he will speak to one of our new main charcters (Kylo, Rey or Finn).

In conclusion I think that they won't just crudly explain away Anakin Skywalkers absence In the film's....on the contrary I think he will have very short but significant roles in both the Anthology and Sequel Trilogys.