Mark Hamill reveals early in development on Star Wars: The Last Jedi, he inquired about the possibility of saving Luke Skywalker's heroic sacrifice for Episode IX. The actor's feelings on Rian Johnson's movie are well-documented by now, with his comments about fundamentally disagreeing with Luke's portrayal gaining notoriety months before the film was released. In the time since, Hamill has expressed regret over publicly sharing his concerns, stating that creative differences are common in filmmaking. His stance on The Last Jedi now is that it is a great movie and he loved the experience.
Still, this dynamic will forever be fascinating in the Star Wars community, since few people have a better understanding of Luke than Hamill. It shouldn't come as any surprise this topic is touched upon in the behind-the-scenes documentary, The Director and the Jedi (available on the Blu-ray), and while most of the discourse is material already covered, there was one new aspect to be gleaned from the documentary.
At SXSW after a screening of the documentary (via IndieWire), Hamill discussed his pitch for Star Wars 9, which involved keeping Luke alive for the sequel trilogy finale:
My first reaction was, ‘Can’t we push this off until [Episode] 9? Luke eventually does the right thing selflessly for the good of the Rebel Alliance, and [I realized] I should do the selfless thing for the betterment of the movie. Seriously. I’m not trying to be self-aggrandizing, I’m just saying, in the greater scope of things, number one, I never expected to come back, so this is all like found money.
It would have been fun to see Luke live to fight another day, but The Last Jedi is arguably a stronger film with the climax it has. A case can be made there wouldn't have been much for Skywalker to do in Episode IX had he lived. His arc in the sequel trilogy (grumpy curmudgeon to heroic legend) plays out over the course of The Last Jedi, so there may not have been a compelling conflict there. Keeping Luke around would have dragged his role out longer in a series that really isn't his story. Yes, The Last Jedi title is a direct reference to Luke, but the sequels are about Rey and Kylo Ren's respective journeys. Now, Episode IX can focus on the two youngsters without worrying about giving Luke his due. One of the reasons why Skywalker was saved for a last-scene cameo in Force Awakens is because he overshadowed the newcomers.
Plus, it's likely we haven't seen the last of Hamill in Star Wars. Viewers would be quite surprised if Luke did not return as a Force ghost in Episode IX to share some wisdom with Rey at a time of need (or, in a twist, haunt Kylo Ren). Such an inclusion would have to be organic to the story J.J. Abrams and Chris Terrio are telling, of course, but with Leia no longer a part of Episode IX, Rey will need someone with experience she can go to. Nobody should expect Force ghost Luke to start electrocuting gorilla walkers on the battlefield, but perhaps he can finally give his gifted apprentice that third lesson he promised on Ahch-To.