Something occured to me that I haven’t really thought of before:
The episode What Lies Beneath, where the Student 6 face their fears, has scenarios that could have been fleshed out into fantastic episodes on their own.
The three that spring to mind are Ocellus’s fear of the changeling’s bad past, Silver Stream’s fear of the Storm King, and Sandbar’s fear of disappointing his idols.
For Ocellus, this could have been a great episode, where we delve into guilt and how it can affect us. An episode really putting into light how some changelings have trouble forgiving themselves for their past actions. Sort of similar to Do Princesses Dream of Magic Sheep, but with two main differences: A fear of relapse, and guilt for collective actions rather than that of an individual.
The Silver Stream episode also could have been powerful if done right, basically delving into PTSD. If her speech was at the climax of an episode of buildup rather than as a small piece of a larger ep, it could have been much stronger.
Sandbar just needed some character development and definition. If he had an ep that cut past his easy-going exterior, and showed that sometimes he pushes himself too hard out of a fear of failing those he respects the most. Heck, could have a great moment where Applejack has a talk with him that she had gone through something similar herself, and relating to him what she learned.
The Student 6 had a lot of opportunities to delve into really interesting topics, themes, and morals, as well as further exploring non-Equestrian culture, but it was squandered, and that will always bug me.