Luke dying would be cliché at this point. And anyway, what's the point of becoming a jedi if your life expectancy is cut in half? Where's the karmic justice in that? And since we have seen the mentor bite it a bazillion times already, it wouldn't even be very shocking or surprising. Have Luke die while he is 150 and while watching sundown. If you ask me, Han Solo is the one to die. His death would be much more unexpected and could be made meaningful too, especially if his daughter is the protagonist.