I think your examples are very bad, and that the examples make me think that we aren't talking about the same things. Hense, I could give an answer, but it would be meaningless to you.
Specifically, what's wrong with your example is that Han has more screen time in 'Empire' than in 'A New Hope'. So how can the story be less about him and more about Luke? The only difference is that by Empire, Luke was beginning to realize that he had always been central to the story - whereas in 'A New Hope' he didn't know that he was.
But moving on, there are several questions here, and the fact that you (and persumably Monte) are confusing the various questions make it hard to answer.
1) Are the PC's central to their own story?
This is actually a far more important question than the question of whether the PC's are central to the story of the world. The PC's can be at the center of the campaign (that is to say 'the game'), even if the story of the campaign is only a small thread in the story of the world. The real question is how much are the PC's actors in the story, and how much are they merely observers who are along for the ride. Is the story about the PC's, or is the story about some NPC's that the PC's know. Leaving aside the question of which goes on in anyone's campaign, I've seen published modules that are examples of both cases. For example, some Call of Cthullu modules seemed more concerned with reproducing an HPL short story than actually giving the PC's anything to do. The PC's merely were observers to the strange events going on around them. To me, that was completely silly. On the otherhand, if done well, then having a narrative in which the PC's are slipping in and out of can be really cool provided that the PC's actually have something to do and that enough temporal paths (rooms) exist in the story (dungeon) to provide a sense of non-linearity.
2) Can the PC's choose thier own story?
This is one of the touchier issues in role playing. Many players just hate being railroaded. They don't want to fill that all thier choices have already been made for them by the DM. Most DM's are sympathetic to that, but preparing a quality adventure is alot of work and full freedom on the part of the players makes for alot of DM headaches. Generally speaking, cooperating with the DM and doing the story he wants you to do is more satisfying to most players than merely going about randomly killing things - at least by sometime in high school. So there is this dynamic tension where the DM tries to give the players enough slack to go off in direction that he doesn't expect, while at the same time keeping the story on track. The players on the other hand have to cooperate enough that RP actually occurs, while still being something other than a mere prop in the DM's story. Things can go off the rails at either end.
It's taken me a long time to realize this as a DM, but it is a necessary precondition of role playing that the player characters be the sort of characters that are willing to take risks and get into trouble. I use to beat my head against the wall trying to make my hooks compelling enough that they would compell characters with basically no reason to go out and risk thier lives to actually go out and risk thier lives. I've come to see that this isn't really my problem but a failure on the part of the PC's to actually play a character that participates in adventures. I now make it a condition of introducing a PC that the PC has some sufficient motivation for wanting to do dangerous stuff.
Incidently, I've been guilty of this myself. I can think of one published adventure I just blew because I played it like it was 'Tomb of Horrors' and I was so concerned about not dying that I could not be hooked into participating. Partly that is the fault of the published adventure for not giving enough clues as to what the rational approach was, but partly that's because I was busy trying to save my own bacon rather than being a hero. Fact of the matter is that if you as a PC have consented to play the part of a hero just by setting down to the table, and so you might as well get on with it.
3) Is the PC's story central to the world's story?
If the PC's are 1st level, then unless one of them is Rand Al'Thor or some equivalent character, the answer probably should be no. And even if the character is Rand Al'Thor, then the player of that 1st level character probably shouldn't yet realize that his story is central to the world's story.
On the other hand, if the PC's are 20th level, if they are not yet central to the world's story, then IMO something is seriously wrong with your campaign. Persumably, the sort of challenges that are actually challenges to 20th level characters don't exist over the hill from every hamlet in the realms, else how in the world would the poor commoners survive? Dire rats in the cellar of the hovel are within the bounds of believability, but if thier are nests of elder wyrms in the basement - why is thier still a village much less a hovel? Instead, 20+ CR challenges are persumably rare things that threaten the safety of the world or at least very large parts of it, and its only by having 20th level characters going and facing those challenges that the campaign world doesn't turn into something more like the Midnight setting (or worse).
By 20th level, the characters aren't just local heroes, they are super-heroes and like any super-hero it is thier job to save the world (or at least a large portion of it). Any thing less would be a waste of thier talents.