Princess Luna is, to me, one of the most interesting characters on the show. There is simply so much potential for stories regarding this character that she's pretty much a gift to the series.
A character who rose up to try and overthrow her sister and claim rulership of the world, only to be put down and banished for 1000 years. Freed from captivity and eventually returned to the side of light. There are so many things you can do with that.
Explore what it was like for her to be imporsoned for a millenium. See her fall from grace. Find out what the relationship between the two sisters is like when the other characters aren't there. And much more.
There's also the issue of her evil alter-ego, Nightmare Moon. Was it simply a conscious choice to become evil? Was it some other evil force taking her over? If so she either reefls regret for her actions over an active choice she made, or resentment towards her sister for imprisoning her when it wasn't her fault.
Is that character of Nightmare Moon and actual separate character or is it actually some dark part of Luna's own personality that has simply been locked away?
And that's just off the top of my head. Who knows what seasoned writers could do with this character.
Anyway, sorry for ranting like that. Good picture.