I know this is a while ago, but I figured I would chime in here right quick:
When I start a new campaign, I usually start a new campaign world to go with it. I know people will say... WOW that is a great deal of work. But in fact, it does not take a long time to create a new and unique living world that grows with the party. For me, the creation of a new gaming world is to provide a framework for the party to experience and enjoy themselves. By NOT defining everything from day one, you leave yourself with many areas to 'fill' in the blanks with.
What I do as a general rule is review modules, old issues of Dragon magazine, etc... for broad campaign ideas. Or come up with something unique.
In a campaign that started 10 years ago (it has now finished with a different rotating DM). I had a cool concept that the world was starting to slowly freeze. Ten years ago, the world was more tropical but now the temperature is slowly dropping. It was going to be a back ground event until they finally (after traveling and hearing rumors) discovered that there was an ice island near the equator. They had to investigate that (after level 10).
I laid the early groundwork for a Gith invasion (from a Dragon Magazine article).
Placed a few of my more favorite modules in the world so they would exist if I ended up DMing w/out having time to prepare something custom.
Determine if there will be any wars in the regions... the location of a few nearby dragons for background flavor in the inn. And create a list of 20 or so rumors (true and false) to drop in the inns. Some may not be used for years, but it will build depth in a campaign.
And setup a list of 10-15 re-occurring NPC's and Villains that will be long term in the campaign. This list was just a starting point and as the campaign evolved they picked up different enemies and/or befriended a previous villain.
The point I am trying to make is don't define everything in detail more than a few sessions in advance. Lay groundwork for the big epic adventures, but let the party guide themselves along and let them build the world for you by THEIR actions.
I had an adventure setup where they had to meet a Baron. They did not know they were going to run into him, but they were just traveling the world. When they got to a crossroads, it really did not matter which direction they took... they were going to run into the adventure hook. All I had to do was move the location of his fiefdom to fit on the road to the N,E, or S on the map with a quick note.
If I had a fully developed world, I would of had to lead the party along a specific road to get that adventure started.
So the last thing I would like to say... Give the party at least the illusion of free will whenever possible. The easiest way to do that is to not set your campaign into stone and always have a backup adventure if they don't take the bait.
2007 & 2008 DCC Tourney Champion