Original Rainbow Dash

I just got the book My Little Pony: The Art of Equestria, and I must say it's a fascinating read. One thing stood out for me though.

In Ms. Faust's original ideas and sketches for the Mane Six, we have these tidbits on page 25:

"Sometimes Rainbow Dash is aloof and brooding—no one knows why."

and...

"

...sometimes she disappears for days at a time."

...with accompanying drawings. I would have loved to have seen these aspects of Rainbow explored in the show, but I suppose it was considered too dark for it. Still, it makes me wonder where she was going with this line of thought.