I came about some interesting snippets from the blog of Mark Rosewater, head designer of the well know Magic: the Gathering card game. A common criticism among long time fans of the game is that as the game became more and more popular it moved away from the creative character designs and art styles found in the game's early artwork and settled on mainstream fantasy ideas which aren't really different from any other generic fantasy setting.
Adopting a mainstream style is understandable and was probably unavoidable for a game which didn't have a coherent lore from the start and grew to become one of the most popular games in the world, but other choices made by the designers don't make a lot of sense to me. For example in the game there is a kind of creatures known as "Slivers", which in their first few iterations were hive-minded insect-like creatures which granted special powers to each other where there were several of them in play.
In their most recent iteration though the art department abandoned that unique and very recognizable design, settling for a humanoid form which can't be told apart from a number of other generic humanoid monsters or even from the Eldrazi species developed in the latest expansions.
Comparing the game's current art with its early art there is an obvious attempt to represent a wider variety of human races, but the overall diversity of visuals in the game has decreased quite a lot. Now all the main storyline characters except for two are either humans or the Star Trek kind of aliens, that is humans with fancy makeup and some weird body feature. There are no more whimsical species introduced just for the sake of creativity or charmingly weird artwork such as that of Kaja and Phil Foglio, Ian Miller and a number of other early Magic artists.
Many of the creatures portrayed in early artwork didn't even feature in the game at all. Which kind of evil overlord would hire a giant bunny servant? Nowadays any marketing department would be mad about that and say it's stupid and alienates players, but actually when it happened it felt quite natural and was a big part of the aesthetic appeal of the game. It gave the impression that there was much more going on in the game's world than what was shown on the cards: they were just glimpses of a bigger picture. I feel this impression is vital in depicting a lively world as opposed to a simply coherent one, and it is something that the current practice of worldbuilding is extremely bad at with its almost exclusive focus on coherence and economy of ideas.
These are some of Rosewater's answers to questions about the change in Magic's design philosophy:
I would very much like to see some of these studies, but I guess he's talking about biased self-limiting studies such as those based on focus groups because those statements are clearly false in the general case (and the assumption that most creatures in the game must be "relatable" is frankly ridiculous, who would ever want to relate to a Sliver whatever its design?).
Characters don't need to be 99% human in order to be relatable, it all depends on the context. If we are talking about realistic psychological drama then yes, human characters are usually the most relatable. A book like Anna Karenina would be just confusing if the characters were aliens or anthropomorphic animals (even though I'm sure some fans of postmodernism would love it). But on the other hand books like Redwall or Incandescence can only work because the characters sport some degree of "otherness" from humans. The brand of My Little Pony: Friendship is Magic managed to capture millions of grown ups on top of its target audience of small children simply because the characters are very endearing and relatable as simplified cartoon horses.
But even sticking to mainstream fantasy stuff: the Lord of the Rings tries to convey enough feeling of realism that it would only work well with humanlike characters. Yet The Hobbit doesn't take itself nearly as seriously and I'm quite sure it would work just as well if the hobbits were replaced by anthropomorphic rabbits or something like that. (Tolkien always denied that the concept of "hobbit" had anything to do with "rabbit", although there is plenty of evidence that he associated the two, as pointed out by Douglas A. Anderson in his annotated edition of the book.) The ever growing popularity of furry art is a clue that anthropomorphic animals may be even more relatable than humans in the right context, for example when portraying sexuality in a slightly idealized way.
In other words we related to believable human characters but we are also eager to relate to unbelievable non-human characters. It's character design as a whole that matters for relatability, the interplay between psychological complexity and physical design of characters.
Such interplay can't be easily translated to formulas though, so it's not surprising that it's left to individual creativity and that a corporation trying to sell a game would rather choose limiting but safe formulas. Plus there are other factors at work, like the decrepit cultural convention that grown ups must only like gritty "realistic" stuff (which usually isn't realistic at all, but feels so because it's gritty and pessimistic). Still it's always annoying for me to see "only human characters are relatable" repeated as a mantra.