I think figuring out what will keep my players interested is the sometimes the hardest part of the role. I've rarely gone into a game with a rigid expectation of what it will be, and it always seems to evolve as we go. I suppose if I started with a firm vision of what the campaign would be like, I would also be sure to discuss up front with the group to make sure they were bought in. I kind of did that with my most recent game, establishing parameters on how lethal the game might be, whether they were into dungeon-crawling, and such. But as I mentioned, it has certainly evolved and morphed over time.
In fact, as I think about it, one of the strengths of a long-running campaign is that you can mix up the sessions. Just because you are in the midst of a mega-dungeon doesn't mean that you can't have a mini adventure arc where the party signs up with the Dungeon Hall Enforcement Squad and do a police procedural for a few sessions. I do things like that from time to time, and generally tease the idea before fully investing.
All of that is a long-winded way of saying that in my mind the game is cooperative, and part of my role is to find scenarios we will all enjoy.