What is the main drive of your campaign? Share this with your players.
What are the six most important things you want your players to understand about the world and the campaign? Share these with your players.
Who or what are the three major drivers in this campaign? Often these are villains. You don't want to share this with your players. Instead, create a villain card for each villain. Add a major goal for each villain and three sub-goals that take them to that goal. Keep these villains to yourself.
What are the specifics of character creation for this world? What are the character building limits in this world? What house rules are in play? Share these with your players.
Who are the patrons and factions the characters can be a part of? What brings the characters together to adventure in this campaign? Share these with your players.
Discuss what safety tools you and your players are comfortable using during this campaign. I recommend including the following safety tools: