There are many kinds of campaigns you can run and many ways to describe a campaign. However, people tend to have certain ideas, themes, or enemies they prefer to use. One of my earlier posts said the same thing, and I know I'm guilty of it. Undead are just so much fun. However, it can also be incredibly rewarding to break out of our comfort zones and do something new. It can be difficult, that's for sure, but the benefits are often worth it.
This kind of thing can be a bit risky. The hope is that by going outside your comfort zone, you can get game experiences you otherwise wouldn't have. Such a thing can be particularly important when you hit a wall or start getting bored with running games. However, I find it works best if you really want to go after that new experience. It can be very hard to do things you've never done before and having the resolve and desire to do so goes a long way. It makes it seem less intimidating. Doing it in a responsible manner is very important as a Dungeon Master as well. Both sides of the table should be having fun.
One of the most effective ways to get outside my comfort zone for a campaign is to play around with the mastermind behind the events opposing the players. Typically, this kind of character influences the campaign in incredibly large ways based on who they are even if it's taking place in the same campaign location. What kind of henchmen do they have? Where are they? What's their plan? How do they like to achieve their goals? How do they act when they have the upper hand compared to being bested? All of these details stem from the adversary of the campaign and can go a long way to inspiring something you might not have thought of otherwise. It's also one of those things where we often have a favourite type and need to fight the urge to repeat the same kind of villain.
On the other hand we can start with a setting first. Particular settings can inspire and lend themselves particular kinds of villains. What that particular kind is can vary based on the Dungeon Master, but in general I've found it works very well. Even looking at published adventures, I tend to be drawn to different kinds of encounters and ideas when running a Ravenloft campaign compared to a Planescape campaign. It generally doesn't come from general appearance, though sometimes the appearance can go a long way to help come up with the rest. A village in a prairie region inspires very different ideas than being in hell itself. However, in these cases the appearance aids coming up with ideas and rules for the world. It's those rules and ideas that tend to have the biggest effect on me. It's not uncommon for the rules I come up with to influence the physical characteristics.
There are many different rules we can think of and not all will apply to every setting. However, some general ones come up fairly often. Can the players escape? In Ravenloft, the rules of the world typically try to trap them there with the horrors. How and to where can players travel? Cities? Dimensions? Other planets? By spaceship or by portal? The other major one that often comes up is on how magic is treated. Are spell-casters generally accepted and are magic items common? A bad guy afraid of death and going to any lengths for a potion of longevity makes far more sense in a low magic world than in a world where you can buy one on any corner for a silver. And of course, such a choice can result in certain themes and situations to arise that otherwise you wouldn't have thought of. A potion of clairvoyance can also sometimes be used as the motivation as well, particularly if the bad guy wants to find something.
It's very easy to have every campaign in the same kind of level of danger. Either resurrection magic is plentiful, it's rare, or it's somewhere in the middle. Regardless of what the preference is, I've found that Dungeon Masters typically stick to one. This can be fine. It tends to be the level of danger we are good at. However, playing with this element of a campaign can result in big changes to the feeling of the campaign. It can also influence the previous 2 elements if it's core enough. An occasional harder combat encounter is one thing. No healing magic at all is another. Playing with this idea can lead to some interesting situations. A deadlier game lends itself far better to horror type games. A less deadly one can be more easily played for laughs, since players will need to optimize their actions and characters nearly as much.
Tone is a hard one to talk about. Still, the general impression you want for your campaign can help give inspiration for the previous things we mention. If you want a darker campaign with an emphasis on dark humour, your ideas will be different. Approaching the campaign with that mindset tends to lead me with different ideas. On the other hand, if I want a more light hearted campaign, it leads me to others. Some ideas can be played in both ways. A colony of mind flayers or a creature that takes over a person (from my experience this tends to be some form of ghost or an intellect devourer, though a doppleganger can also be used in similar ways) can be incorporated in both types of games. The horror aspect is fairly obvious but the totally obvious strange guy with the party that the players always dismiss due to increasingly silly reasoning is an idea I've seen used before for comedic effect as well. One thing I want to note here is that sometimes being aware of the tone you want to go after can make a big difference subconsciously and actively.
Personally, I find that the first step is often the hardest. Get the ball rolling and things get easier. Good luck to you all.