(In a recent EN World thread , I offered some advice for running an “organic” campaign – a campaign where players can go in any direction, finding adventure wherever they go. Here’s what I wrote.)
Some ideas for this kind of organic campaign: You need to present foes that come from other places (a hook), are looking for a certain item (another hook), are too strong for the party so they need to look for an ally (another hook), villains that manage to escape and return to fight another day (makes them feel at home), a safe place to return to and defend (home sweet home), etc.
Basically I think it is very important that you don’t waste your energy early in the game. Have two or three small things ready that you can spontaneously place anywhere when the party goes in unexpected directions. Reuse people and places. Hometowns, returning villains, old ruins reoccupied, that kind of stuff.
Some examples from my current campaign to illustrate these ideas:
The important part is that I haven’t prepared anything about this valley, yet. But I’ve planted powerful cues that adventure is to be had there, and should the party turn there, I’ll start working on it. For now I’ve spend time detailing the bandit castle, adding turtle demons (kappa), a long lost magic weapon by a powerful demonhunter, an evil cleric helping the bandits, trying to ursup the position as band leader and founding an evil cult. (Haven’t decided on his masters, yet.)
And so on. Plant hooks, and don’t be afraid to reuse people and places. Don’t work too far ahead.
(And, as I found out later, this is old advice: “Never force yourself to create more than you must.” – Dungeoncraft, from Dragon Magazine 256, by Ray Winninger )