The PCs in my homebrew campaign are very important.
I got fed up with fantasy worlds which had tag lines like: "In the great days of yore, when gods and mighty mages strode across the veldt, wielding power too great for the ordinary mortal....blah blah blah..."
I want the PCs in my world to be those who gouge the eye out of vecna, or be vecna, even. I want them to possibly create the items that will persist for a millenia and become the artifacts of the future. I want them to cut the ring from sauron's finger, or tear down his forge before it can be made. Granted, they aren't epic level, but if they get there it will be a blast, and they'll get to shape the world.
In this world, the elves and dwarves are young races, meaning they haven't been around any longer than humans. Magic is just coming into prominence and the climate has been such that all of the human/humanoid/demi-human races are going through huge population explosions. War is on the horizon. Everything the players do changes the landscape, and I play off of it. I have general ideas on how the main NPCs behave, but much of their "plans" change as the PCs cause change throughout the game world.