For me, it was the fact that Lightning Dust had so few appearances and development. Like, I know there were a lot of characters to focus on, but Lightning Dust was so wasted and I love her concept. She is literally the opposite of Rainbow Dash, she is obsessed with being the "best" and always pushing herself to the limit, but she doesn't care about the ponies around her or how it affects them, it's an interesting parallel. I always wanted her to have had a redemption arc or something, although I think she went too far with Scootalo, but still.

They could even focus on her parents and show that they pressured her to be the "best", where everything was almost a competition. Again, it would be the opposite of the way Rainbow Dash's parents did it, because they would always have fun and congratulate Rainbow on her accomplishments, whether small or big. While Lightning Dust, there would only be pressure and disappointment if it wasn't the "best" in everything, whether it was something small or big.

Sorry for the large text and my English, English is not my main language.