I've had this idea for a Rarity episode for a long time.
The premise is that Rarity starts feeling jealous of Twilight and all that the latter has been given and has accomplished (i.e. grew up in Canterlot, her great talent in magic, being Celestia's apprentice, becoming a princess, great leadership skills, her wisdom and insight on the subject of friendship, etc.). So Rarity tries to learn new spells and do bigger things and solve more problems, always approaching the problems by asking herself "what would Twilight do?" I imagine she also might confide in her pony friends or ask them for advice, most likely Applejack and Fluttershy. But still, she can't perform the new spells as well as Twilight, who already knows most of the ones that are new to Rarity, and so Rarity is still not as helpful as Twilight, making her feel more hopeless about it. At the climax of the episode, a new problem arises, one that has been built up in the background throughout the episode, and for some reason, Twilight is unable to fix the problem on her own, or is unavailable for it, so Rarity ultimatly comes in to fix it. I imagine another of the mane six (possibly Pinkie) would go up to Rarity and say something like, "Forget what Twilight would do! What would RARITY do?" And so then by the end of the episode, Rarity learns to not judge yourself based on comparisons to others; be proud of who you are and what you can do.
That's the basic outline. What do you think?