"Save the World" seems way too often just meaning to keep the status quo instead of getting to a worse state. But maybe the "trick" is to change the status.
1) Make the world a better place.
Get rid of some evil or injustice. I don't know about 4E Forgotten Realms, but if it still contains the Wall of the Faithless, that's exactly the kind of "end-game" I could see - bring it down. The world doesn't get safer (hey, it might get worse - who knows what the faithless souls will do?), but you remove a (percieved) injustice.
Alternatively, you kill a major demon lord (for good - keeping them contained for 100 years is stuff for the heroic or epic tier

), or even Asmodeus himself.
2) Change the Rules of the World.
For example - you don't like the fact that the gods can meddle in mortal affairs. Remove them from the equation - banish them, slay them, or just force them into a deal. Nobody knows if the world is getting better by it.
Or you don't like souls getting "lost" after death - ensure that all souls will be given the option to reincarnate.
3) Widen the World.
Only very few people have the ability to move the Feywild/Shadowfell/Astral Plane. Change it. Create a way so that anyone can will himself to go there.
Or alternatively, there are other material worlds out there - find a way to permanently open a portal to one of them, so that the other world(s) can be explored.
4) Create Something Big
Found a new nation, or better yet, a union of nations that will oversee the world to do whatever you want them to do - bring world peace, gnome genocide or what-you-have.
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One of the hardest parts is to figure out how to resolve the final steps to the non-world-saving. The standard model - especially in D&D - is to end a plot with a combat against the BBEG or whatever he summoned or allied with. But combat hasn't to be the only option.
1) Bringing down the Wall or a Demon Lord can end with two big armies (with the PCs in the lead of one) fighting it out. But it could also end with a major skill challenge where the PCs have to convince the gods that the wall has to go (whether the PCs just convince them "morally" or by saying that the alternative is that they kill a few of them before they go down might depend on how the campaign went so far and how the skill challenge works out)
2) Changing the Rules could require the party to reach a mystical (and warded) place where they can make the change. They might have to kill or subdue the Raven Queen, or they have to complete a major ritual in the mystical place - or both.
3) Find the "Keeper of the Gate" that controls all interplanar travel, and kill/subdue/convince him. Or again, perform a ritual.
4) The party might first need to win a major war. Or they just have to get in contact with all major rulers, get them to a meeting and convince them of their proposal (skill challenge/role-playing time).