I ran a lot of ideas through my mind until I finally settled on this. I really liked the Student 6 when they were introduced, and Gallus really won my sympathy and respect in The Hearth's Warming Club. So, I was pretty excited to see, in the time-skip in The Last Problem, that he had become the captain of Princess Twilight's Royal Guard. From an orphan on the streets of Griffonstone to the leader of the Princess of Equestria's royal guard. Well done, buddy. =)
That being said, I don't remember there being any indication or hints within the show of that being what Gallus wanted to be when he left school. Then again, none of the Student 6 really gave any hints like that. Still, with Gallus being the laidback smart-aleck that he is, I can imagine him daydreaming in class about being a royal guard. =P
Also, I know these prompts call for ponies, but I couldn't think of a pony for this one, so I hope the prompt isn't too strict about that, lol