RPG plots usually center around the idea of saving something before it is lost. Generically somehow it's tied with saving the world before it is destroyed. You are that shining ray of hope for the entire world. And really, I don't think the RPG audience ever gets bored of it. How empowering it is to have the fate of the WORLD tied to YOU.
Yet as a storytelling device, it wants something more...something that makes it believable. Here are some tips to carry in your pocket.
1) It's a Small World After All
In an RPG you can travel the world in a matter of minutes. Making it a very small space. You may get to meet all the populated areas IN the world within the game. That way when you say you are saving the WHOLE world, it is accurate because you have traveled all of it and know almost everyone. Yet, this is unrealistic as far as the narrative is concerned. It takes 3 long novels for Frodo to reach Mount Doom and he didn't even travel the world and meet everyone.
However, it's all a matter of perspective. To a mouse living in a hole in the wall, the whole house may be their world. It is just as empowering to be a mouse trying to save their world (prevent a house from being destroyed) as it is to be a human trying to save a planet.
2) Save the World In chunks
Leveling right? Makes sense! The characters world can expand farther and farther as they save more and more bits of it. Maybe making friends along the way. :)
3) Make it Personal
The most important thing to do is to somehow invoke empathy from the player. They should WANT to save this world and this character. To do this you must give them human qualities.
Do NOT destroy their village, players are now not sympathetic to that idea at all.
Kidnapping is a good one, although also common.
Mystery is another one. Something out of the ordinary has appeared. This draws the players curiosity about "what happens next".
4) Are we really saving the world?
It is a challenge that can be posed to the player and/or the story - is this the right way to do things? Can we really save the world like this?
Usually this is a weak argument in RPG games because you fought all the way there, the system did not allow any other way for you to progress, so how DARE the STORY blame you for doing things that way. Really! How impertinent! *shakes fist*
Obviously this means creatively putting that choice within the game. Kind of like fable - save the world...or doom it yourself! Mwahahahaha!
5) Saving the world from non-destruction
Destroying the world or the end of the world is old sauce. Really making it work takes art and skill. It'd be great if I could think of a way but I really can't so here's what you can do alternatively. Not every villain is out for destruction...some just want to change the world for the worse. So free slaves, put an end to dictators, rob from the rich to feed the poor, or save the last Unicorn. :3
These are all suggestions for making your 'save the world' story for your RPG feel LESS like a 'save the world story'. It's not a bad plot line, it's just been used in many places and as a designer or writer, you should push yourself.