Years ago I played in a game where the GM asked us to help creating the area around our starting town. We brainstormed landmarks, terrain types, enemies, allies, secret societies, and treasures. Sure, our starting area was a little rough, but it was full of cool stuff, and everyone had at least a few things they were excited to explore.
 Then we showed up to play. The GM started the session by explaining that he “didn’t like” what everyone had come up with, and instead started us in an area populated with Twilightish vampires, WoW’s dark iron dwarves, and whatever the hell that green guy from Dragonball Z is.
OK, I admit it wasn’t as much of a disaster as that would imply and the game, while short lived, was fun. I wasn’t mad about it but I was disappointed, both that I didn’t get to play with the suggestions we made, and that our input had been so quickly rejected. It would have been fun not only to see and play with our contributions, but to watch the GM riff off them and take them in his own direction.
This brings us to the “No shit Sherlock.” moment for this article; if you solicit input from your players, use it. If you get more than you need, you don’t have to use everything, but make sure you at least use some of what each player suggests. Use similar amounts from each player and make some of everyone’s highly visible. If there’s anything more annoying to a player who suggested campaign material than having it simply ignored, it’s having it ignored or hidden away in a lost dungeon somewhere while another player’s suggestion is used as the central theme of the entire game.
Now that you know to make use of player input that you receive (as if you didn’t already), let’s look at common forms of player input. The special part is that you don’t really have to solicit it. Almost everything about the PCs and every action the players take is a window into their designs for your game if you know how to read it right.
PC stats are the first and most basic level the players give input to the game world. It’s not a hard and fast rule, but generally, players select skill sets for their characters based on what kind of challenges they think will be fun. In addition, they choose character options that they think are cool, and mechanical bonuses to support the kinds of things they want to be able to do. This means if you’ve got a player with a Dwarven socialite with bonuses against lizardfolk, your player thinks dwarves, social challenges, and beating down lizardfolk are awesome, and wishes very much you’d include those things in your game. Of course, if they just took a dwarf for the Con bonus, the socialite class for the skill package, and the lizardfolk bonuses as a step towards a prestige class, then it doesn’t mean that at all, so you have to exercise your judgment as to what these things are really saying.
The next step up is PC backgrounds. Not only do these have the impact of calling out the kinds of things your player wants to see in game, but they’re rarely muddled in mechanical bonuses the way stats are, so they’re easier to trust. Even better, backgrounds allow the player a measure of narrative control to introduce their own elements into your game. The mysterious brotherhood, peasant rebellion, or space station featured in a PC background are full of meaty goodness for your campaign. Unfortunately, even if required PC backgrounds are often minimal or skipped entirely.
Sometimes you can solicit player suggestions or are just given them. When a player asks to fight martians, that’s your cue to bring on the martians! Unlike other sources, these are almost all signal. The issue is that different players will provide different volumes of suggestion, both in terms of amount and insistence. Thus it can be difficult to strike a balance between the player who won’t stop giving you suggestions and the one that has to be harassed into giving them.
While you can watch the PCs actions as indications for what they want to do, it’s often difficult to extrapolate especially if your game gives them a limited ability to be proactive or seize narrative control. However, by giving them sets of choices and allowing them to choose one, you can help focus on the kinds of encounters or elements they’d like to see in your game. If they always pass on the social encounters in favor of the combat ones, or they repeatedly reject jobs to investigate hauntings, you know those aren’t high on their list of things they’d like to see or do.
If you happen to have a player who writes fanfics of your game (which isn’t common, but also isn’t unheard of) you can’t necessarily assume the things featured in their stories are things they want featured in your game, but these stories are also excellent sources to mine for elements for your game.
In short, keep your eyes and mind open for what your players want to see in your game and use what you find to make everyone’s experience better.