Building A Campaign
Again, the easiest way to do this is not to bother—just let your scenarios and arcs emergently create a story for the campaign. Human beings are pattern-making machines, and it’s very likely that you’ll naturally pick up on what the long-term plot devices of your campaign need to be by keying into unanswered questions from the arcs and scenarios.
However, if you want to do a little bit of focused planning, the advice is the same as for arcs, except you’re generalizing even more. Pick one story question to answer, which the PCs will spend their scenarios and arcs building to. Then, jot down some notes on what steps will lead to answering that question, so you have material for arcs and scenarios.
The very best aspects to look at for a campaign-level problem are your setting’s current or impending issues, because of their scope.
Amanda knows that her campaign will hinge on resolving The Doom that Is to Come. So the story question following from that is pretty obvious. “Can the PCs avert, prevent, or mitigate the prophesied doom?”
She knows that to do that, they’re first going to need to figure out which of the Cult of Tranquility’s factions is right about the prophecy (if either). They’ll also need to make sure that none of their personal enemies or The Scar Triad can interfere with whatever they need to do in order to stop the doom. That gives her a good idea of what arcs are going to make up the campaign.