Some genre series that come to mind that work in the 20-30 minute windows, while animated would be Star Wars Clone Wars, Stars Wars Rebels, Samurai Jack.
I'm a big fan of Steven Universe. Each episode is roughly 11 minutes without commercials. The big difference, though, is it's a running series. Even in a self-contained episode, it's still a series where you keep getting these small bite-sized-chunks of world-building. It adds up over time. (SU is B5 like where there's a mix of arc-centric and standalone stories.)
The guidelines prohibit fan-films from doing this. So you have one shots. From the first frame, you introduce all the characters to the audience for the very first time, and you say goodbye to them forever after 30 minutes (with some sort of awkward part 1 part 2 split in the middle).
It can be done but it is an altogether different kind of thing vs. the examples cited (including Have Gun Will Travel where Paladin was the recurring character).