The biggest thing to keep in mind when you're starting down the path of putting something together for a group is Time vs. Player Agency. The shorter the game you're planning on, the less agency you're probably going to give your players. Think of your average module or Adventurer's League scenario or whatever. They tend to be one-session games; by the end of four hours or so, the party has solved the problem at the heart of the scenario. There's really no room there for a player to say "My character is going to go off and search for his father's magic sword" with essentially writing that character out of the rest of the session. So if you're planning something that's designed to fit a certain timeline, be aware of how much of that timeline should be your material, and how much should be their material. By comparison, for something that you envision running long-term (maybe a year or longer, depending on how often you play), let the PCs go nuts. Request backstories, and mine them for adventure ideas. If you have a character whose goal in life is searching for his father's magic sword, write something that lasts for a few sessions, maybe not the sword itself, but tracking down someone who knows where it is, or the means to disable the "bad side" of its enchantment, or whatever. You can get the other players to go along with that one person's B plot, because 1) they're the players, and that's sort of implied in the contract, and 2) you can lace those few sessions with opportunities for them to grow, and to spin off into their own sub-plots as well. The players will write the game for you if they have enough time to accomplish their characters' goals, at which point you can let them do the hard work, and you just keep yourself ready to improvise any material you might need. Feel free to reach out to me if you want to discuss more. I've been running games for a while now, and have sort of developed my own set of guidelines for these sorts of things.
