One of the first decisions you must make when planning a campaign, is if you are going to use the existing setting from the rulebooks, or are you going to “homebrew” an original setting.
Using the Source Material is certainly going to result in a lot less work for the DM, as you have maps, history, and all sorts of pre-existing information ready to use. This would likely be most helpful for a new DM, or one who doesn`t have a great deal of time to work on their own original setting.
Homebrewing a setting can be attractive though, especially for the DM, as it lends itself much more to telling a uniquely personal story. And a way a homebrew setting can really shine, is that it taps into one of the best aspects of playing a roleplaying game: unique experiences for the players.
Players love to have stories to tell that are unique to them. There are only so many variations of crawling through a dungeon and fighting some zombies. But a running battle across rooftops against a cabal of thieves led by a sorcerous werewolf? That’s something the players who lived through it may remember and be able to talk about for years.
Sometimes when reading through a particularly good bit of source material, an idea may come to you. “Yeah, that’s a cool bit of lore there, but I wonder what would happen if…that powerful wizard had an evil twin!”
There is absolutely no reason he couldn’t have a twin. Creating a side story to attach to an existing one can be a nice middle ground between using the source material, and creating a completely unique setting all of your own.
You don’t always have to create your own sandbox, sometimes you can just make some new sandcastle in an existing sandbox (and this metaphor is getting convoluted now so I am going to stop).
One of the worse things to happen with a homebrewed world is it just be a generic world with a funny hat tacked on. By this I mean, have the one thing that you’ve made unique about this world seem like an afterthought. Picture a generic fantasy world, where every other sentence people say is “oh, and fish is currency, don’t forget”.
If you are committed to the idea of a homebrew with a special twist, but you want it to feel like an actual living, breathing world, you’re going to have to work it into the very fabric of the setting. Really consider how it would permeate the daily lives of the people who live there, and let that ripple outwards, and you’d be surprised how organically a world can evolve when you start thinking about it.
For example, a world where magic is viewed as inherently evil? Is divine magic exempt from this prejudice? Would this create tension between users of the divine and arcane practitioners? Is there a now a black-market for arcane items, and are they simply illegal, or illicit? Would the nation go to war with other nations over this belief? Have they done so in the past? What effect would such a war have had on their history? These questions and the answers you create for them can go on as long as you need them to.