Campaign in already existing world and story – fear of NPC domination


This is my first time running a campaign, with just some one-shots of experience.

I thought it would be easier for me to create a campaign in an already existing world that I know very well (World of Warcraft). However, since I would like for my players (who do not know the story at all) to experience the story and to follow some important characters, I fear that they might see their story and aventures as "secondary".

I definitely will allow them to change the story, apart from a few cases which I want to keep.

Do you have any tips on how to allow my players to experience the story arc of some important characters without giving them the feeling that it might not be "their" story and that they are not just spectating some NPCs?