Getting back to planning a game as a Dungeon Master is fantastic. Part of the joy of running a game is that you get to see people directly react to your writing, and then add to it. It's a storytelling conversation, that's really an art form. It's improv with statistics and stricter structure.
By the way, if any of you visit Chicago and you have a chance, do the walking tour at Second City. It's definitely fun and funny and you learn some things about Chicago and improv as an art form. Did that yesterday with a college friend and it was fun. Sorry about that interruption. Back to the RPG talk!
The writing skills you need to create a great game for your players are a little different than the ones used for a standard fiction piece. As a writer you should create a linear path for your reader to follow as your main characters go through a hardship and overcome it. But when the person enjoy your story is also producing the main character of your story, there are a couple of things you have to consider when writing. These two rules tend to guide me, and a couple of friends of mine who also run games, into making a game a fun venture for everybody.
1. You must know where you want to go.
My friend who runs my Obsidian game always reiterates this tip. Specifically he says "If you run the scene you're in without knowing where you want it to end, you have already failed." He's absolutely right. In college we though having an open game without railroading the players meant presenting 1 piece of a puzzle and expecting the players to build the rest. But with any game, you have to establish a thread for the players to follow. Whether they know it or not, most players spend the game looking for that thread.
Games that don't do this dwindle fast. When I first started running my D&D day, I built in days where nothing would happen thinking my PCs would go do their own thing those days. But most of the times my players were just waiting for something to happen. As the DM/GM/ST you are in charge of the action, and you know what's coming. Your players don't. Sure, you may want to give them some time to investigate something, and some games you know for certain that they will, so you'll set aside that time. But you can't assume they'll have motivations of their own every single game, especially when you first start. Set up that thread for your players to follow. They are trying to see where it is, and will at least like to know where it's going.
However, at the same time,
2. The players will decided where you really go.
You have to be ready to set up a ton of things for the players to do, but you also have to be ready to be directed by their responses. Just because you think that your players will hold on to one clue or be captivated by one situation doesn't mean that they absolutely will. They may find a certain NPC worthy having as a friend or a lover. They may decide that certain monsters can be trained for good. They may even decide to start a taco stand on the side. You will never be able to 100% predict what a character my want to do. And you don't want to deny them something that they'll find fun.
The great thing about this is that this process is how you can add to the thread that you start. Sure, you may have wanted to have your players explore the catacombs of the city, but certainly you could tie in your plot points for when they try to raid the local sheriff's office. Maybe that NPC noble your players like to antagonize is actually helping them out where they least suspect it. The players pursuits are a tool for writing the plot. You have to be flexible enough to go with the flow.
But why should you make a path that your players are going to veer from anyway? Because then you know better is is off the beaten path. If you know what you would like to have happen, then you have a better idea of what will happen if your players make other choices. And that helps you more if they through you a curve ball. I've had many a times where characters pursue flavor text, but because I had at least made the time to include flavor text, it was easier for me to think up something on the fly. That helps game flow and consistency, which make playing in a particular universe more fun.
In the end, you still have to write a story with a path, but willing to continue in a different direction when the players introduce it. That makes the most successful RPG campaigns fun and memorable.