I already have an idea for how he is as a pony, too; prim and proper and every bit the prince that he is, however he has a couple of self-esteem issues. This is mainly because he has a condition where his horn is stunted in its growth, resulting in a short stub of a horn (the condition sometimes calls unicorns that have it "hornless unicorns") and the inability to wield magic through it. Due to this he had to learn how to release his magic in a different way, and he does so in a way earth ponies do; through the hooves. While this suits him fine it makes him self-conscious that he's the only one in his family that doesn't have a working horn, and while his family tries to let him know that it's nothing to be ashamed of he still feels this way all the same. He'll eventually overcome it and gain more confidence in himself, specifically through the power of make-up!
Dusk Shine's the type of guy who loves putting on make-up and dressing up, and boy he's really good at it too. Despite being a prince with the responsibility of the kingdom in his future, Shining and Cadance want their children to have the freedom to be who they are and what they want to be. In this case, Dusk would have probably been a make-up artist otherwise; as a foal Flurry Heart taught him how to apply make-up when she caught him playing in hers, and let him practice on her until he bettered himself at it. To this day, Flurry Heart would seek her brother out to do her make-up instead of doing it herself.
Fun Fact about Dusk Shine: He's often mistaken as female but it doesn't bother him in the slightest.
As for Flurry Heart, I've actually headcanoned her as perhaps the future captain of the Crystal Guard. As a youngster she was very much a rambunctious gal; all over the place and daring and giving her father and mother heart attacks left and right. Where's Dusk Shine was a Momma's boy, Flurry's a Daddy's girl, and given her father's past on being the captain of the Royal Guard in Equestria, then the Captain of the Crystal Guard, I can see her wanting to be just like him when she get's older. :3
Anyway I'll just leave it at that for now. Feel free to ask me any questions if you have any, and thanks so much for takin' a look! :3
You: "*Where's* Dusk Shine was a Momma's boy, Flurry's a Daddy's girl, and given her father's past on being the captain of the Royal Guard in Equestria, then the Captain of the Crystal Guard, I can see her wanting to be just like him when she *get's* older. :3" (* = Typo)
I like the concept of going off another family member's coat color cause in real life traits are passed down from one generation to the next and sometimes traits are from another family member not just the mother and father. I don't know how genetics work in Equestria