I got asked this question a few times during my early days on this wiki. Having seen Tanks for the Memories and a few other episodes that portray Dashie in a negative light this idea came up and made me think of one conclusion...
Would Rainbow Dash make a better Villain than Hero?
I'm asking you this because I feel if she started off as a villain rather than a bearer of harmony she could have gone through some redemption process which would have made a few episodes (particularly Dragonshy) more understandable. Do you really think if she was an Anti-Villain that she would be more rounded? That more people would like her like Sunset Shimmer if she started off as a general rude bully character (like Gilda and Diamond Tiara) but slowly softened up to characters like Fluttershy, Scootaloo, and so on? In one episode it could be revealed her father was a perfectionist and taking his daughter to the Equestria Games in Cloudsdale was his only caring act. Maybe Rainbow Dash started as a weak flier and was taunted because of that, but gained social status and felt guilty for bullying Fluttershy as a foal. Would these changes to her character make her more interesting? Or would she be considerably less likable than she is now? It's implied that Dashie may have accidentally participated in the bullying of Fluttershy but stood up for her in the end. I also got this idea from those who said she doesn't deserve her element, which made me think "If Rainbow Dash doesn't deserve to have an element of harmony, wouldn't she be a better villain?"
So tell me what you all think in the comments, vote on this poll, etcetera. I'd talk more but I don't really know what else to say other than would this make her a more interesting character and more three dimensional and flawed. Like she had to earn her element or something.