I've started with the setting, main story overview, and major design elements (character system, combat, multiple ways to handle quests, non-linearity, etc). I didn't do all the quests or ways to solve them, of course. I'm talking about the concept phase here. For example, one way to keep the main quest non-linear is to have multiple factions interested in it. These factions shouldn't want the same thing as that would make your choice of a faction less important. So, they should want different things which would give you different reasons to pursue the main quest and would require you to make very different decisions once you are at the end of your journey. At the same time you can't be sent to several different directions at once, so your final destination should be able to offer and support different outcomes, etc. In other words, you develop the frame of a game first and then fill it in with the actual content.​