My Little Pony: Friendship is Magic - Image #262,981

I know some people would disagree, but I think Gilda should have a redemption episode more than any other antagonist.
My main reason: So far, she and Rainbow Dash are the only friends in the series that have split up.
There's a great lesson in there about giving people a second chance, not holding grudges, 'no friend is truly lost if they are willing to change', etc.
I think it could work quite well.