Twilight Sparkle: My main concern is obviously her new status as an Alicorn Princess. I'm still not 100% sold on this new role for Twilight because not only did I feel there was still much Twilight could do before ultimately taking this next step, but there's so much room for error now that Twilight's role has changed. The fast pace of MMC felt a little bit too sudden and out of nowhere for a change that could be really dramatic if not handled properly. I want Twilight Sparkle's ascension to princesshood to be one that develops gradually and naturally. There's plenty of doors that are open now that Twilight's character has had a change in status quo, and I just hope that these potentials will be handled with proper care. I've already warmed up to the concept. After all, Twilight did have to work to become a princess, which is something we don't often see in fiction aimed at younger girls. There's a lot of good messages to be had, I just don't want Twilight to ever stop being Twilight. She's always been a very well-balanced protagonist that I've enjoyed watching as she's grown, and I hope that I will be able to say the same when watching her grow as a princess.
Pinkie Pie: As others have stated, Pinkie Pie was more or less reduced to little more than a joke character in season 3. I think Pinkie Pie is the best source of humor on the show, but that's not her only purpose. During the course of season 1, Pinkie Pie was constantly proving herself useful even in her usually unorthodox ways. Her inspiring song about sharing and caring, her knowledge on how to send away parasprites, and of course, her pinkie sense. A lot of what made Pinkie Pie such a fun character in season 1 was her random silly songs which were peppered well throughout all of the first season. However, more recent episodes have failed to utilize what seemed like an important aspect to Pinkie's character, unless of course the episode was focused on her (Baby Cakes and A Friend in Deed). She went from being the pony who laughed in the face of danger to being the pony who ran around like a chicken with its head cut off where there was danger. Pinkie's no dummy. I want to see her use her head more often.
Rainbow Dash: Dashie has come a long way. In season 2, I was really rubbed the wrong way at times due to her excessive narcissism, but in season 3 we got to see more of her sweet and sensitive side, which is something that Dash needs to become more comfortable in expressing. Many of Rainbow Dash's episodes have had her focusing too heavily on her "reputation" in one way or another. It seems to be the biggest factor in her acting in the way that she does. I would love to see Rainbow Dash finally overcome her fears of her rep getting ruined, and just be more genuine with herself and others. She'd be a lot more at ease with showing her softer and more warm side. Other than that, i just want to see her take some more steps towards achieving her dream of becoming a Wonderbolt, but I don't want that story arc to become too fast paced either.
Rarity: My main gripe with Rarity is that her negative qualities tend to be exacerbated for the sake of comedy. I know that she's vain, and that she is very much a drama queen, but these negative traits are shown a lot more frequently than any of her good qualities. The fact that the writers had to rewrite Spike At Your Service from a Rarity episode to an Applejack episode due to Rarity sounding very unlikable tells me that the writers have a lot of difficulty in balancing out her strengths and weaknesses. Rarity is perhaps the most complex character of the six, but there's still many good qualities to her that seem to be overlooked or inconsistent, namely her generosity from time to time. Rarity has an innate sense of creativity that I'd like to see more of as well.
Fluttershy: It feels like whenever Fluttershy overcomes a major hurdle, she trips over the next one. Her character weaknesses are perhaps the most pronounced and detrimental, but it feels like there's been far too little progress or growth in her character. What growth there is, is terribly inconsistent. I was very disappointed to see Fluttershy so upset about Twilight using her magic on them in Magic Duel. We've seen Twilight turn her mice into horses, and Fluttershy was perfectly fine with that. The writers need to find the right balance of strength without totally taking away what makes her Fluttershy. There are times when Fluttershy feels a bit too much like a shtick - she's fearful and weak at first. Then, she overcomes the odds by summoning up her inner strength. I would also like to see more of Fluttershy's backstory and perhaps her family life, because that would help to identify what it is that made her so sheepish.
Applejack: Well for starters, I'd like to see her be in the spotlight just a bit more so as to dispel the background pony notions. I happen to believe that Applejack is very interesting and likable just by being who she is. She doesn't have a story arc, destiny, or aspiration to follow, and she doesn't have a major character weakness to overcome, but she has always been a very lifelike and realistic character both in her lifestyle and personality. Because she is more mature and secure than the others, she grows a little bit more static than them, but she's still plenty capable of learning more lessons and being front and center in episodes, and I want season 4 to prove that. There's too much talk amongst the fandom that she's fully explored or "done" as a character until they expand on her. The only thing that I feel could use some expansion at this point is her dead parents, which I think could make for a great story and help the audience connect with Applejack and further appreciate who she is. Beyond that, I don't think she's the type of character who necessarily needs heavy changes or new turning points because she challenges the conventional storytelling structure of a protagonist accomplishing a long-winded goal/destiny and living happily ever after. Too much expansion would quell her role as being the simplistic and earthy character that she's always been, so I hope they don't try to change AJ too much. If season 4 can just prove that Applejack can be the star of an episode, make even small or subtle changes and progression in her character, and yet still be entertaining as always, then I will be perfectly happy with that.
Oh, sweet Celestia.... What did I write? ^^