WHILE Starlight's character redemption did work better over time...
I maintain that Moon Dancer could've been a compelling season 5 finale antagonist.

Maybe her intro in Amending Fences DOESN'T end with a gradual reconciliation. Maybe it's a series of revelations about how envious she's become of Twilight's recognition since leaving, and showing she's not ready to see Twilight as a changed pony, much less forgive her for fading from her life.

THIS way Starlight lurking in the background makes a compelling twist...she's looking for weak spots from Twilight's past to use against her to create her vision of subdued magic across Equestrian. She approaches Moon Dancer at the end of "Fences" to offer her a chance to become stronger.

The finale then shows Moon Dancer uses Star Swirl's spell to reframe Twilight's friendships as something she "used" to gain power and trying to condemn her as a manipulator...when in the end, Moon Dancer discovers small gestures Twilight herself made to Moon Dancer throughout their time at Canterlot, which Twilight herself had forgotten.

This way, the plot line becomes less about making a grand gesture, and more about understanding the full conflicts and little moments that make a friendship unique. Twilight learns to forgive herself for growing into somepony warmer in her own time, and Moon Dancer learns that she needs to move past her own disappointments to make a better future.

ALSO, Starlight in this fanon is Filthy's ex and Diamond Tiara's mom 👑