I do a lot of writing these days, some professional and some just for fun. Often, I have a vague notion about the topic, but no specifics. I learned back in college that if I just start writing about the topic, eventually my thoughts will crystallize into an argument that I can hang a paper around.
I run my campaigns the same way. The first session of my current Moldvay/Cook/Labyrinth Lord game was back on the 22nd of January. Only this morning, after chewing on it late last night when I couldn't fall asleep, did I finally get around to writing down what the big movers and shakers were doing. And even then, it was in the vaguest of terms.
Why? First, because what the big movers and shakers are up to hasn't been terribly important. The party started out shipwrecked, made their long way back to civilization, and now is involved in tracking down a small party of dangerous elves. Yes, what the Powers That Be are up to does have some influence on this, but it's only tangential. Yes, there will be clues for the PCs to find, but nothing solid yet. The truth is still out there, somewhere, but it's not likely to fall into their laps for some time yet.
Secondly, I wasn't sure what the PCs and players were interested in. And I'm still probably being premature, so I've left things vague. It could be the PCs will, in the end, have nothing to do with the grand schemes I wrote about. It might only end up as background window dressing. If that turns out to be the case, there's no point in me pouring buckets of sweat and blood into it. Much better to focus my energies where they will bring the most fun to our game. Right now, that's a bit of old school dungeon delving mingled with some para-anthropology and empathetic NPCs.
Third, I was still getting a good feel for this campaign. Nearly every other D&D game I've run in the past has been high fantasy of some stripe or another. This is the first where I've tried to inject a bit more Bronze and Iron Age feel. Plus, the new rules for magic and character classes might still alter the flavor of the game in ways I haven't anticipated yet.
And finally, the players are still putting their mark on the campaign. The players always add their own spin on things, and until you do you can never really tell what new dimensions or angles they might bring into focus. It's their game, too, and should reflect their interests and desires as much as mine.
It all comes down, eventually, to wiggle-room. You always want to leave a bit of room for a new monster, or an adventure in an unusual environment, or a sudden shift in the interests of the group. A good campaign is a dynamic one, and keeping things loose and malleable at the beginning gives you lots of room to define things as you and the players express your interests in the play of the game.