Mane 6 has changed over time, their weaknesses are diminishing after they learned new things. the writer can not reuse the moral they will have to find new lessons. I think, the more developed character = less topic to tap on the character. Twilight was perfectly developed = less focus on her, Rainbow Dash still can exploit ..., more potential in SS5 Fluttershy, Pinkie Pie still has many aspects can be exploited (Her sentiment is still much mystery ),Applejack = back ground pony , rarity is nicer now = less focus. Season 5 now need the undeveloped characters much like Discord, Luna, Celestia, CMC, Trixie, Diamond Tiara, Gilda and so on. Bring more mature, dark subjects, some twists, to keep the show fresh... maybe some villain episodes too =]].