A mysterious asteroid flew across a time rift, entered the Earth's upper atmosphere, and crashed into the Pacific Ocean. Its sudden appearance updates the physical fundamental constants.
Now there is a novel quantum field called "magica" that permeates the universe; seemingly individuals who have a high "affinity" can cause an excitation of the field with their will alone. The problem is that this magica comes in 5 different types: Metal, Wood, Water, Fire and Earth.
It seems that magica behaves like a set of tuning forks: each generates an unique frequency but will switch between the types from time to time. So far, so good, but I don't want everybody to be able to produce all the types and also it doesn't look good if a user has to wait for a certain amount of time to elapse to get the desirable result, so what better rule can I introduce to perfect this magica oscillation? Of course it must also allow a few extremely gifted individuals to be able to stand out from the crowd by utilizing more than 1 type of magica simultaneously.
Simpler version to understand magica oscillation phenomenon:
Imagine the aftermath of an asteroid impact on Earth gives rise to a novel quantum field called magical field, it is can be excited by our thoughts and manifest itself into 5 different types depending on the magical mixing. Imagine there are many tuning forks with their unique natural frequency and there are only 5 beats or types in this case due to the interference in their harmonic frequency, now I don't want everyone to couple with this quantum field and I also like only a handful to be able to manifest at least 2 types of magica by introducing a few rules or minor changes on the existing ones.