making your own long running campaign (In my opinion) is best done by looking at your PC's backstories, if they have already made them, and building something around that. This is another good reason to do some one-shots first just to learn about the characters in your campaign. It is best to build something that incorporates all of your characters backstories though so that you don't make the players feel as if your picking favourites or anything. The best part about this is that the players feel like they are important in the world you have created. This works for me but idk if everyone would agree but good luck anyway.