Draw Inspiration from Literature
Heroic fantasy literature provides a wellspring of inspiration for D&D campaigns
and dungeon masters can use their favorite fictional settings and characters to develop campaigns. For example, a Conan-inspired campaign could begin with players contending with situations appropriate for low-level characters, such as doing battle against Setian cultists. As the campaign progresses, the players should logically come to the attention of major heroes and antagonists, and the alliances or enmities they forge will provide direction for continuing adventures. Let the action progress naturally and don't be afraid to deviate from the events depicted in your source material if your players are taking you in a more interesting direction.
Explore Different Genres
You don't have to confine yourself strictly to heroic fantasy when you're designing your campaign. D&D lends itself well to mystery and espionage-based campaigns, allowing you to craft individual adventures where players contend with locked room mysteries or delicate intelligence gathering within a conclave of nobles. This campaign type provides ample opportunities for role-playing, as players try to coax confessions from suspects or finesse information from courtiers. D&D easily adapts to various subgenres of horror as well. Dungeon masters can set a tone of understated menace with ghostly or vampiric antagonists lurking within a township, or they can thrust players into a survival horror scenario, pitting players against a zombie apocalypse as they desperately try to stay alive while protecting survivors who may or may not be infected themselves.
Take a Walk in the Woods
Give your players a change of scene by setting your campaign within a vast, uncharted forest. You can populate your arboreal campaign world with competing societies of elves, nomadic gypsies and other denizens with whom players could form alliances or do battle against. A forest setting also allows you to create encounters that give specialist classes a chance to shine. Rangers and druids have the opportunity to use their tracking and nature-based magic abilities to their fullest, while ensuring that mainstays, such as fighters and mages, have no shortage of challenges tailored to their skill-sets.
Think Globally
Dungeon masters can add novelty to a campaign by choosing a setting other than a familiar medieval European backdrop. Drawing inspiration from Chinese, Mayan and other world cultures less frequently represented in D&D campaigns allows you to draw from a rich repository of civilizations, deities and creatures that will be more likely to spark excitement in your players than yet another tussle with marauding orcs. Have your players adventure on African soil, tasking them to gather the eggs of the legendary lightning bird or set your campaign in ancient China and have your players contend with the fallout of a war between the demons from rival precincts within the region's stratified and bureaucratic version of Hell. Presenting your players with exotic nemeses and locales makes your campaign distinctive and heightens your players' anticipation for the campaign's next challenges.