There is absolutely nothing wrong with the general, sword and sorcery high fantasy world that so many DnD games take place in. But, sometimes it's nice to shake things up, and keeping things fresh is important to avoid boredom at your table. So, in the name of inspiration, here are some ideas for thematic worlds to use in your next campaign.
1. A World Without Gods
It's not that the gods' existence is in doubt, or that they never existed, but for whatever reason they seem to have stopped responding. The legends and lore of the gods exists and is still worshipped, but divine favors, miracles, physical appearances; all these things have almost completely disappeared in the last 100 years. In fact, it seems as though only the most legendary and powerful figures are able to tap into holy power at all...
2. Technologically Advanced
Many worlds we play in have existed for millennia, and yet they still seem only to have unlocked the metallurgy of the middle ages. What if there was some more modern technology? Like phones or cars or waterslides? This doesn't negate the existence of magic or gods, and probably shouldn't extend to modern weaponry, but it would be fun to play around in a world where you have both technology and elemental power at your fingertips.
3. Flat World
If the Discworld series is any indication, a classic high fantasy with the biggest difference being the fact that it is flat is incredibly entertaining. It brings into question where water goes and comes from, what's on the underside of the world, how gravity works, as well as providing for some fun regarding conspiracy theorist NPCs claiming the world is actually round.
Everyone loves the wild, crazy outdoors, but the city's a jungle too. Picture a druid who can summon wild beasts despite being in the heart of a thriving metropolis with no greenspace. Buildings that house thousand year old trees, but only by pumping oxygen in through magical tubes leading to a different plane of existence. Districts that vary widely, different races building different styles of buildings, city permits playing a huge role in the political makeup of the city.
5. Water World
Generally, planets are primarily made of land. But, what if they were mostly water? It can be fresh or salt, but there are plenty of items that allow consistent and safe travel through water, plus with the UA rules expanding sailing and boats you can feature naval movement and combat. Now, the world should probably still have some kind of land, but then solid land becomes a valuable resource.
6. Literally Hell
With the number of terrible people present in these terrifying magical worlds, it is perhaps safe to assume that hell is chock full of souls. That's a lot of people to look after, especially for a group of lazy, rebellious devils. So, put the players in a hellscape with some oversighted overlords and let them run wild. Are they trying to break out of hell? Take it over? Establish a chain of ice cream stores? Who knows, but hell presents a challenge for any normal activity that makes the campaign fun.
7. A Universally Held Ideal
Apply a single ideal to the entirety of your world. For instance: greed is good. Take that one thought and expand it to something that every single person believes and agrees with. How does that effect cities, culture, governance? How do people react when someone breaks from that universally held ideal? There are plenty of ideals that we as a real society don't adhere to universally, but imagine if we did. Imagine if we didn't believe in the existence of "up". Imagine if strength was the most important value. Imagine if nobody lied.
It is very important to communicate these themes to your players before character creation, since some of them will effect how classes or races would be played. But, with so many cool worlds and world ideas to base your campaign in sometimes its nice to shake it up. Don't be afraid of taking risks, for risk is the butter within which creativity cooks.