"I can't believe you saved all of Equestria" said Twilight
"Well that's just one of the perks of being a princess, I stepped down from the throne because I was too powerful, but I still have all my powers" said Cassandra, "For I am the seventh element of Harmony, Trust."
"I understand," said Twilight, "I should have trusted that you could save Equestria, thank you for teaching me this lesson. You should hang out with us and go on our adventures."

Author's Notes:

For those of you who think Cassandra's a Mary Sue, you couldn't be further from the truth, she has conflicts like any other character. For instance there's a storyline where Rainbow Dash and Rarity both confess their feelings for her, and she can't decide which one to choose.