Rule 8 of 24 · Chapter II — Systems and Their Price
Let the system have side effects
Why this rule exists
A power that does exactly one thing and nothing else feels like a tool bought for the plot; a power that leaks, that changes the people and places around it, feels like part of a living world. If magic exists, someone has tried to farm with it, wage war with it, cheat with it, and get rich on it, and each of those attempts leaves marks the story can use. The side effects are where a system stops being a mechanic and starts being a culture, an economy, a source of prejudice and hope. Readers register this as depth: the sense that the fantastic element did not arrive yesterday and touch only the hero, but has been reshaping the world for a long time.
In practice
Take each power in your world and ask the ordinary questions a bureaucrat or a farmer would ask: who profits, who is put out of work, what does it do to prices, to armies, to who marries whom, to how the poor are treated. Follow it into daily life, the second-order effects on trade, law, faith, and status, and let those show up in the texture of scenes rather than in lectures. Look especially for the ugly consequences, since a power that has only upsides is a power no one has really lived with. The world will feel inhabited to the exact degree that its wonders have side effects someone has had to cope with.
When it doesn't apply
A tightly focused story may deliberately keep the wider ripples off-page to hold its lens on one thread, and that is a choice, not a failure. Fable and parable often strip the world to a single clean mechanism on purpose. But if you want the world to feel large and inhabited, the side effects are not optional decoration; they are the inhabitance itself.