I think I’ve said it somewhere on this site before but Cadence and Twilight compliment each other well if you’re familiar with the four loves. Cadence represents romantic and familial love. Twilight represents love for your friends and for humanity (“a stranger is just a friend you haven’t met yet” and all that jazz).
As for Celestia and Luna, there’s a bit of a “God and Lucifer” dynamic, what with the whole “fallen” thing. Given that Luna was eventually given dominion over dreams and bat ponies making an appearance, I think if that was part of her character from the start we’d have a completely different story. Probably something closer to a civil war than just a fall from grace.