How do I catch the second-order consequences of my design decisions?

You can make sound design decisions now. The trouble is the second-order consequences, the effects of a choice that only surface later, once the system has grown around it. That is one of the hardest things in the craft, and struggling with it is not a sign of weakness. Nobody sees every downstream effect in advance. The best simply get better at spotting more of them sooner. Second-order consequences are hard because they hide in the future and in the connections between parts. A decision that is clean on its own can turn costly once other things depend on it. You cannot foresee all of them, and trying to will freeze you. But you can widen how far ahead and how far sideways you look before you commit, and you can build your systems so that a wrong call is easier to change later. Before your next significant decision, pause and ask one question: if this is here in a year and everything is built on top of it, what will I wish I had thought about? Then, wherever you can, favor choices that are reversible over ones that lock you in. You will never catch every consequence. You can catch more by looking further, and you can soften the ones you miss by staying able to change your mind.