It would take stunning buckets of power to create a magnet field the size of an arena, that’s capable of suspending a person (and broom). But the real difficulty is in coming up with a way of targeting 15 separate objects: two teams of 7 players, and the “snitch”. In theory, you could put together some kind of extremely sophisticated array of small coils in the ground and in the stands that could target a magnetic field to relatively small areas in space (person sized). In order to get a wave phenomena (sound waves, ocean waves, magnetic, whatever) to stay stay confined (like a beam) and not just go wherever, you need the wavelength to be fairly short. Real-world Quidditch would a very short, but spectacular game.