Quick Answer: When Bob's Burgers was originally pitched to FOX, the Belchers were a family of cannibals. However, FOX deemed this to be too dark for an animated sitcom and removed that idea. Other changes include cruder animation and the absence of Tina Belcher.
Before becoming the show that we all know and love today, the original pitch for Bob's Burgers (2011- ) was noticeably different. When Loren Bouchard first pitched the cartoon to FOX, the Belchers were a family of cannibals who used human flesh to make the burgers for their restaurant.
However, FOX deemed this to be too dark for an animated sitcom. In an interview with A.V. Club, creator Loren Bouchard said, "There was a brief period when I first started talking to [FOX]—because I was coming off of doing Lucy: Daughter of the Devil and I had this kind of occult-y, sort of darker edge to the way I was thinking then — I did pitch the show [as] a family of cannibals who runs a restaurant. There's sort of a Sweeney Todd aspect to it. They basically said 'everything but the cannibalism,' and that included the cast."
Bob's Burgers Episode, "Human Flesh"
Although the Belchers are no longer cannibals, this storyline was retained for the pilot episode of the series, "Human Flesh." In the episode, health inspector Hugo closes down Bob's Burgers due to rumors that the burgers contain human remains from the funeral home and crematorium next door. Though the rumors prove to be false, the episode is a nod to the original pitch for the series.
There are several other notable differences in the early demo pilot for the series. Not surprisingly, the animation and character designs are cruder than the images seen in the series today. In terms of narrative, the most glaring difference is the absence of the eldest daughter, Tina Belcher. Instead of Tina, we are introduced to the Belchers' eldest son, Daniel. Interestingly, both Tina and Daniel are voiced by the same voice actor, Dan Mintz. Although Tina and Daniel have similar character designs and personalities, FOX was worried that Daniel may have been too similar to Gene, the Belcher's youngest son. So Loren Bouchard pitched the idea of changing Daniel to a girl, who would still be voiced by Dan Mintz. FOX agreed, and this led to the creation of Tina Belcher.
The Belcher family (with Tina's predecessor, Daniel) from the early demo pilot
The Belcher family (with Tina) from the episode "Human Flesh"
This change, motivated by a practical concern, arguably resulted in a stroke of creative brilliance. Tina is now a widely beloved character whose androgynous voice adds personality and charm to her textured identity as a boy-crazy yet sensitive young teen who still loves ponies and has erotic dreams about zombies.
Tina isn't the only female Bob's Burgers character to be voiced by a man. Linda Belcher, Tina's mom, is voiced by John Roberts, who was nominated for a Primetime Emmy for Outstanding Character Voice-Over Performance. Through making Tina and Linda voiced by male voice actors, Bob's Burgers has created two memorable female leads that challenge our gender assumptions and distinguish the show with unique, unexpected characterizations. Although it would have been interesting to see a darker take with a cannibalistic version of the Belchers, Bob's Burgers is one of the most entertaining and acclaimed animated series on television today.