Throughout the show we've seen Twilight Sparkle, Rarity, and Rainbow Dash come a very long way and are well on their way to realizing their goals and ambitions.
Rarity rose from being an unknown, small-town tailor to a rich and famous fashionista.
Rainbow Dash is swiftly rising as a new Wonderbolt and is impressing the bigwigs at every turn, she's clearly got an amazing career Wonderbolt ahead of her.
And of course, Twilight rose from being an everyday commoner to a princess based entirely on her own merit, not genealogy.
But when it comes to Applejack, Pinkie Pie and Fluttershy, well...they're just sort of there. What have they accomplished in the last 5 years in terms of their own lives? They never really seemed to have any life ambitions like Twilight, Rarity and Rainbow Dash did, being more content with their own lives as they already are. It feels like half the main cast is being pushed to support roles, having little in the way of their own goals.
Applejack in particular I have to sympathize for. I mean, she had originally wanted to make the rural flight and see more of the world than the farm she was born on. She went on a pilgrimage as a child to the city to get a taste for more of the world, but then here cutie mark was just like "LOLNOPE" and sent her right back to the farm, and there she's remained. She seems like her life goals like with family rather than wealth or fame, but if that's the case then she and Big Mac are doing a pretty terrible job of keeping their lineage going. I mean, Applejack is in her late 20s and Big Mac is like 30. If they want to continue their bloodline then one or both of them better start looking for a mate soon.
Regardless, these 3 are sorely lacking in personal character development. I feel like they're unfortunately getting sidelined in favor of the other 3. I hope season 6 addresses their personal stories...or lack thereof.