Characters That Need to Come Back!
This guy got, like, five lines in "The Enchiridion", yet he was one of the funniest parts of that episode. Who exactly is he? Where did he come from? How did he get "The Enchiridion"? Do we really need another backstory episode? YES! With all that being said, I feel like he's a legitimately funny character who could definitely return in a future episode and it would not feel forced in the slightest.
Ricardio is a despicable character. One part genius, one part sociopath, and one part misogynistic ass, Ricardio is just a great person to hate. "Ricardio the Heart Guy" was weird and funny, but his all-time best episode has to be "Lady & Peebles". That final confrontation with PB was amazing; she left him in a pretty bad shape. I'd love to see him return as the antagonist for another episode, since George Takei's signature voice is so fun, but also so effective.
What more is there to say than just this: Martin Olson. As many of you probably know, good ol' Hunson Abadeer is voiced by the one and only father of Olivia Olson (i.e. the voice of Marceline). That in and of itself is pretty neat. With that being said, every episode that Hunson Abadeer has been in (all four of them) have been hoots! I have a feeling (sorry, no official confirmation) that we'll be seeing him in the miniseries that is set to air soon. We'll see.
Goliad was the perfect marrying of weirdly unsettling and childly innocent. Her one and only episode also ended in such a way that a future story was, while not assured, at least not completely impossible. In the season four DVD commentary for "Goliad", Pen Ward explained that Graham Linehan was penning a story outline for the two. I have no clue if this outline ever made it past the 'draf't stages, but if it has, then we may very well be in for a future Goliad episode!