Well, Fluttershy is a character that might be often in 'My Little Pony weight gain fanfiction'. Given her gentle and shy personality, a change in her body due to weight gain could bring out new aspects of her character, like how she deals with the attention it might bring. Rarity is another one. Her focus on appearance and beauty could make for an interesting story when her body changes. And of course, Applejack, with her strong work ethic on the farm, could face different challenges if her weight changes due to various reasons like changes in her diet or less physical activity.
Definitely Twilight Sparkle. Her role as a leader and a learner makes her a prime candidate for stories about weight gain. Any changes in her physical state could impact her leadership and her studies. Pinkie Pie is a no - brainer too. Her carefree and food - loving nature can easily lead to a weight gain storyline. Also, Spike could be involved. He's always around Twilight and his reactions to her or other ponies' weight gain could add an interesting dynamic. And let's not forget the Cutie Mark Crusaders. Their young and impressionable selves could be used to tell stories about body image and weight gain in a more educational or heartwarming way.
Twilight Sparkle is often involved. As a central character in the My Little Pony universe, her studious and sometimes self - conscious nature can be interesting to explore in the context of weight gain. Pinkie Pie could also be a main character as her love for parties and food might lead to some scenarios of weight gain. And Rainbow Dash, with her athletic build, could have an interesting character arc if she were to experience weight gain, perhaps struggling with her identity as an athlete.