Hopefully we'll get ghost Luke in episode 9. I don't look at it like Luke died really. I look at it as he saved the rebels and it was his time to rest. Just like Yoda had trained Luke and then it was his time to rest. Of course Yoda was like 400 years old and Luke lived to like his mid 50's, lol. He was all forced out.
Take Obi-wan. Those of us that were alive before the prequels were introduced to Obi-wan in ep 4 and an hour later he's dead. We didn't know much about him and weren't invested in him so it wasn't a big loss. But If you watch the movies in order starting with episode 1, its different. You are attached to Obi-wan and in ep 4 he meets his demise. There is more weight with his death. Same with Luke. Watching the movies in order of 1-8...he has a life cycle of about 4 episodes, just like Obi-wan.
After ep 9, we will have a trilogy of trilogies and Luke's existence is in the middle. It's more than just about him now. There are others that come and go and only R2 and 3po will probably be in all of the episodes.