Interesting theory. It helps with their unique talents(not the rising of the moon/sun thing)

Celestia has precognition ability, meaning to some extend she can see the future. Thus from her eyes everything is predestined.

Luna on the other hand has the ability to see and interact with dreams. Dreams are usually crazy and unpredictable but also clear: Imagination. No matter what you see in your dream, it is just a dream.



Normally one would think Celestia is right and thinking everything is predestined, however my own personal headcanon is that the tree of harmony actually directing the world a bit to where it wants it to be heading. Meaning Twilight was chosen to be the element of harmony because she reflected the necessary traits the tree was looking for.