I know the idea was explored somewhat with the side quest against the changlings with Discord, Trixie, and Starlight, but I always thought it would be good to have an episode where the mane 6 were absent from Ponyville (off fighting baddies somewhere). Then, some major problem would come up. Some background mares (background 6, maybe?) take it upon themselves to solve the problem in ways that turn out to be unique, cute, wholesome, and funny, etc. Maybe Amethyst Star could be the leader, since she's the 2nd best organizer in Ponyville.