I started building D&D campaigns when I was 11, which means I've been building campaigns longer than I've been driving (legally, at least). Over the years, I've learned exactly one thing about building campaigns: Don't do all the work yourself. Give your players space to enhance what you create, and when they try to add to your campaign, embrace their ideas as if they were your own. Just say yes.
All kinds of D&D books are aimed squarely at players. Just as I urge you to pilfer campaign ideas from other sources, you should also incite your players to pull ideas from core rulebooks, power sourcebooks, and campaign player's guides. Encourage them to explore and use what's out there.
I dare my players to introduce new elements to my campaigns. I double-dare them. Case in point: When I sat down to build my first 4th Edition campaign (which plays out every Monday and Wednesday night with two different groups of players), I designed the world of Iomandra around the PC races in the Player's Handbook. I gave no thought to how goliaths, gnomes, shifters, and other races might fit into the world, and I didn't go out of my way to create space for them, either. When Greg Bilsland (one of our editors) joined the Wednesday night game, he wanted to try out an early version of the genasi, which hadn't yet been published. I said yes. When a player comes to you and asks to add something new to your campaign, you should also say yes. Why? Because "your" campaign is the players' campaign, too. If one of your players can imagine a place for genasi in the world, then so can you.
When the time came to insinuate "Brell" into the game, Greg came armed with a background for his character. In fleshing out the hows and whys of Brell's past, Greg had questions about genasi in Iomandra. Where do they live? How prevalent are they? Do they have any particular views? What do the other races of the world think of them? My answer to him was basically, "you tell me." Greg wrote a few paragraphs and sent them to me, and I took those ideas and figured out a way to work genasi into the story of the campaign. In fact, I was so inspired by what Greg had done that I decided to turn one of the major paragon-tier villains of the campaign into a genasi. It forced me to make genasi relevant to the campaign, and the campaign is stronger with them around, particularly since Greg and I have portrayed genasi as something of a mystery to the other races.
A campaign world that evolves to include new game elements is much more fun and interesting to players than one that doesn't allow this or that. The trick is to get your players to do some of the heavy lifting, to make them the instigators when it comes to expanding the depth and breadth of the campaign.
Sadly, Brell died before he had the chance to savor the fruits of what he'd wrought. The day after Brell's untimely demise, Greg came by my desk and said, "Is it okay if my next character is a deva?" I, of course, said yes. I didn't have a clue how devas might fit into the campaign, but I basically left that up to Greg. Now it's hard to imagine the Iomandra campaign without them.
What about your campaign? Have you incorporated elements suggested by players which affected the campaign in particular good -- or bad -- ways? Send your stories and opinions to firstname.lastname@example.org.