I am GMing a Warhammer Fantasy Roleplay game right now, and I am doing a mix. WFRP has a very good “Getting Started” adventure that functions as tutorial island, and as the sessions go by more and more of the original script has been replaced by my own campaign. Eventually we will full transition into my own campaign, but the original adventure provides an excellent framework to build off of.

That being said, creating a new campaign wholecloth can be A TON of work, and if you choose to go that route I suggest keeping in mind: noone wants to play your novel, your players want to tell a story together.

You should make sure your players know what kind of campaign you are creating. What the themes and tones are, what sort of adventure they can expect. On the flipside, talk to your players and see what they are looking for in the campaign and what sorts of things should be avoided in the campaign (in the vein of CW type stuff.) Try not to get too caught up on players not doing exactly what you imagine them doing or drawing their own conclusions. GMing and RP is very much like improv, try to stay on-theme while yes-and-ing where your players are heading.