Interesting.

“Where [she] sees a blank canvas, I see King Lear.”

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.

In defense of Celestia, can’t believe I’m using those words, Twilight in particular is a very destiny marked individual. Her cutie mark was part of the Tree of Harmony and the central Element, Magic in particular. I’d hazard a guess that she was somewhat duty bound due to her own previous use of the Elements and Magic itself to oversee Twilight’s life once she saw that mark and to facilitate Twi’s development.
 
Also, well Twilight’s name itself is an obvious sign of her fated importance to Celestia and Luna themselves in particular.

@gittonsxv  
Getting advice doesn’t change your true self. Luna helping them did nothing regarding what cutie marks they got.

Well, if prophetic dreams just intrude upon Celestia’s mind, it’s reasonable for her to feel that way.

@gittonsxv  
Theyll defeat STarlight, I just know it.

@Background Pony #98CE  
well no but i find it strange.  
luna helps the AB and cmc directly in their dreams and gives them advice on cutie marks and what they might mean,  
a bit later on the cmc get their cutie marks but in a very special and unique way.  
it just fits too much for me.

@gittonsxv  

 
well I don’t follow new episodes anymore but does the all dreamwalker Luna episodes/moral from them were mentioned in that new one where CMC got their marks?

Determinism is evil I know it!

apparently when royalty meets you in a void you get special shit.  
with celly she became an alicorn and with luna all of the cmc got special cutie marks.

Interesting to note.