The first rule of world-building is available physics, which basically means that if you want it to feel real, it has to follow the same rules as this world, from gravity to how human behaviour works. If you have a fantasy element that doesn't obey the laws of physics, make sure that it has a fantasy explanation.
'The Magician's Apprentice' was about someone from the low end of society manifesting magical power and how that completely messes up the balance of the whole system.
