Michael Silverbane
Adventurer
Third, if you are going to supply the motive to the story, rather than run a sandbox, start with a bang. If the players aren't already all on the same page, something so big and important needs to happen that responding to it becomes every PC's #1 priority regardless of their motive. And I do mean a big bang. Even if you plan to slow down in chapter 2 or 3 and start building back up with slow rising action, chapter 1 should be huge shattering event of historical importance. Disaster should happen right in front of their eyes. Kings should die, cities should fall, volcanoes should explode, meteors should fall out of the sky and blast half the landscape out of existence, old gods should rise, or whatever. Whatever should happen should be so big and so important that everyone's motive should be survival and dealing with this thing. After that, you can start introducing your plot threads in that context of 'the big thing'.
This is good stuff, right here.
I recall one campaign that I was a player in, started off with riots in the streets and a massive fire ala the Great Fire of London or the Great Chicago Fire. Good times.