Here are some ways that J.J. Abrams can bring Luke Skywalker back as a living, breathing person in Star Wars Episode IX.
I will admit that most of these are stupid. But so was The Last Jedi. And if it takes doing something stupid to get Luke back, so be it.
Listed in no particular order.
1. Rey returns to Ach-to to retrieve Luke's green saber. She arrives to find him alive and waiting for her. He has returned via the Gandalf route. The Force simply sent him back to the living world because his true purpose had yet to be fulfilled.
2. Rey reads the Jedi texts and discovers a Force secret that allows her to bring him back to life. Since his body was made of matter and that matter disappeared, it just needs to reappear now. Some weird Jedi ritual can accomplish this.
3. The Luke that "died" in The Last Jedi was actually a clone of the real Luke. Rey can discover this shocking surprise when she returns to Ach-to to retrieve Luke's other lightsaber.
4. Since Rey is apparently now the most powerful Jedi to ever live, she just suddenly realizes she can summon him back to the living world. Because.
5. Time travel...as presented in Star Wars Rebels. Heck, a lot of The Last Jedi's nonsense can be un-done with this nonsense.
6. Other Force ghosts decide to channel some weird power to bring him back to life. Obi-wan, Qui-gon, Yoda, etc. join hands, close their eyes, and he is magically sent back.
7. Leia and Rey said he was "gone." Gone does not necessarily have to mean dead. He was simply sent to another dimension or "Force realm." Then he returns in the rebellion's most desperate hour to save the day.
8. Leia switches places with Luke. Some dumb, nonsensical ritual is performed and Leia (since Carrie Fisher has passed away) sacrifices herself to bring back her brother. It can be played off as a really noble event where Leia realizes that her Mary Poppins powers are no match for the First Order. They need Luke. So she makes the ultimate sacrifice. Plus, this is a way around the fact that Leia apparently can't die (outer space = no air, negative 300 degree temperatures).
9. Episode VIII was all a dream! Finn wakes up from the bacta tank (or whatever that tube thing is) and we discover it was just a horrible nightmare.
10. Episode VIII was all a Force vision that Luke was having! We pick up right where Rey is handing Luke his saber. The Last Jedi was what WOULD happen if Luke chose to follow the same path. So now he accepts the saber, mourns Han, trains Rey, and then they both go battle Kylo and Snoke.
11. Remake episode VIII and this time don't poop all over Luke's character. He survives and therefore is alive in episode IX.
12. We discover that Ach-to has mysterious powers. When Luke disappeared he became "one with the island" or some garbage like that. Then, he is reborn from the remains of the tree that Yoda destroyed...and his hand is back.
13. We find out that when Jedi pass on, they actually go to some weird realm like Davey Jones' Locker. Rey has to travel to this realm and bring Luke back.
14. Rey and Leia were wrong. He was never really gone or dead. He simply transported himself somewhere else (perhaps on the island). Rey arrives and he's there fishing or milking an alien sea cow or something.
15. Using DNA found on Ach-to, a clone of Luke is made. In order for this to work there would have to be some kind of growth acceleration so that Mark Hamill could play the role.