Game systems often get away with making humans generic. D&D has traditionally made their feature "adaptable" and just given them more of what everyone else already gets. A5E gave humans some unique features, which really helps them stand out. I'm curious about discussing humans from a worldbuilding perspective.
So, speculative fiction often wants to portray an entirely different set of cultures. It makes a world feel more real and larger. This brings some unusual problems because... Well, the fundamental problem with non-humans is that the creators of the games and stories do not have an example of a non-human sapient creature. We don't know what a non-human mind capable of communicating with us and interacting with our society, culture, and civilization on the same levels that we do would be like. Nobody has ever met one in reality. What their cultures would value, how they would organize their society, and how they would live are all entirely beyond conception. We cannot imagine what we are incapable of conceptualizing, and it's difficult to conceptualize something without an example.
Therefore, all fictional "races" or "species" or "ancestries" in any of these games or stories are not actually non-human at all. They're all humans. Everything is taken from humanity or expressed as an absence of some aspect of humanity. That's why I often call them "humans in rubber masks." Like the aliens in Star Trek, all the aliens mysteriously are humanoids about 1-2 meters in height with curiously shaped foreheads and nasal roots.
This kind of opens the question: If they're all just humans, why don't we use humans for everything? Well, some places do. But the advantage of non-human humans is that they're
immediately identifiable as non-humans. They look different and their cultural identity as non-human is not something that can be easily concealed. You don't just "look southeast Asian" or "have Mediterranean features" or "have blonde hair, skin so pale it's pink, and piercing blue eyes" because those aren't recognizable as an individual culture to us. They were at one time in the past, but not anymore, and we don't like viewing humans that way anymore. Even including clothing, jewelry, tattoos, make-up, or other culturally linked styles isn't enough because humans often adopt elements from cultures they don't originate from. Worse, some cultures implicitly or explicitly accept diversity or uniqueness. Humanity is diverse and intermingled, and we want to portray a diverse world of unknown cultures to the viewer, reader, or player... and sometimes even unknown to the characters in the work. So we need it to be as crystal clear to them as it can be that an elf is irrevocably not a human.
So we need to imagine what humans might look like if they were obviously non-human creatures. -- That seems like a strange sentiment, but remember, we have
no earthly idea what true non-human cultures and non-human societies look like. We can only imagine so much, and we cannot imagine what social constructs a truly alien mind would have or how they would differ from us. -- So, as I said, we want to know what non-human humans might look like, while also remaining familiar enough to understand their way of life. So we imagine human-like non-humans in terms of what humans are not. Yes, it really is that silly. So, elves all have pointed ears and delicate features
because humans do not. Dwarves are all short and bearded
because humans are not.
Now that there's an appearance for these non-human humans, we can develop unique cultures for them. What these games want is a set of non-human cultures that have easily justifiable reasons to be unlike the human culture or cultures. Elves live in the forest and are aloof and disinterested in the world
because human cultures are not those things. Dwarves live under mountains, love mining, crafting, and stonework, and value order and clans
because human cultures are not those things.
That's the problem with humans. They're all negative space. That's the real problem. Humans are defined in most campaign settings by
what they are not. That's what you must avoid. The problem is that in many settings, humans are a blank slate and are defined by that blankness.
The easiest example of where to look for how this should work is Lord of the Rings, including the 5e LotR or AiME books. When you pick a "race" in that game, you might pick Durin's Folk, Hobbits of the Shire, Elves or Mirkwood, or Elves of Lothlorien. Or you might pick Men of Gondor or Men of Rohan. We know what the Men of each domain are like. They are
similar to each other, yes, just as the two Elves are. But they have identifiable cultures. If you then go meet the Men of Laketown, they have their own culture, too.
That's the key. You need to have a concrete idea for what
even the human culture is, and I think you need at least two tangible, coherent cultures of humans with completely distinct
cultural characteristics. The way to make humans interesting is to
give them a culture to call their own. Because that's what we're actually picking when we say "race" or "species" or "lineage."
And the default human in 5e D&D? It has no culture. It's culture is "adaptability to any culture" which, while true, is... it's got no story. So, give the humans a story the same way you have for everyone else.
It should be clear that if you see a given structure that you
know a human built it. But you should be able to do that. "Oh, these farms are built in the human style with a farmhouse, barn, small coral, and central well. The fields look like they were wheat or some other human-favored grain." It's
visibly a human style just like stonework would
visibly be dwarven. When the party goes to a human city, you can tell the elf players that they're surprised by how many children they see. "Why, you've seen two or three in just the last five minutes! It's like strolling through a nursery." You tell the dwarf player, "You see stonework and carpentry that, while plain, is simply everywhere and made in that robust and simple human style. It isn't beautiful and it isn't maintained to your standards, but you have to admire the sheer practicality and sturdiness of it all. It's like they know exactly what 'good enough' is."