I would like to see a civilization, reasonably advanced in MTG terms, wherein cannibalism is the norm. Presumably, they only cannibalize criminals, undesirables, or volunteers.
I got a flash of an idea, seeing an Eskimo-like people (note: the term Eskimo encompasses basically
all of the indigenous Arctic peoples, I just learned) who live almost exclusively on sea ice. I would imagine the cannibalism in that case would have started as a means to repurpose the useful resources that the dead body represented (fat for fire, bones for needles and other tools, etc.). If we kind of advance the clock, metaphorically speaking, we could assume that the practice stuck around even as the culture became less dependent on needing said resources, and it would then take on a kind of reverential, ritual quality.
Like, I could imagine from that that, say, a flute made from the bone of a great-great-grandmother would be a treasured family heirloom, or that the cannibalism was largely a family affair, unless they were a dominant public figure like a king or something. Something about "keeping the spirit within the family."
I think Aaarrrgh's mention of "undesirables" not being cannibalized would still stick here, since if the culture is "cannibalism = good," then to not be cannibalized would be like the ultimate banishment. "Your spirit cannot join your ancestors, it must wander the lost lands for eternity."
Personally, I would also go ahead and figure something out about death from diseases and how that would factor in. Like, maybe the race in question has a really good sense of smell that can tell what parts shouldn't be eaten? Or maybe, like the "no pork" rule in Judaism, there's a sort of cultural remnant of practices to not eat certain parts of a diseased person, because way back in ancient times that led to deaths? Organs like livers and kidneys are kind of bad to eat anyway since they naturally filter out wastes and poisons.