In a D&D city, you'll see non-human characters walking the streets side by side with humans. Elves, dwarves, dragonborn, goliaths, and more. This is seemingly not the case in Mythic Europe, where everyon is human.
Or is it ?!
I'd suggest that by the rules something comparable should occur in Mythic Europe's cities, due to supernaturally-blooded characters.
The most common, perhaps is Faerie Blood, which has no visible indication. But those with Strong Faerie Blood do have non-human physical features. Their eyes have an unusual vivid color, and they have a "clearly visible" physical feature. Realms of Power: Faerie provides "some common features" of these characters, which may be taken to be descriptions of particularly potent faerie blood.
* Sidhe-blooded characters are tall and fragile, with pale skin, long blond hair, and pointed ears.
* Dwarf-blooded ones are short of stature, with smooth skin like polished stone, and duck's feet.
* Goblin-blooded are short and twisted, hairless, with a long pointed nose, sharp teeth, and red eyes.
* Satyr-blooded are excessively hairy, with a curly beard, goat's horns, cloven hooves, and yellow eyes.
* Undine-blooded have pale, almost-blue skin, webbed hands, large liquid eyes, and green hair.
To these core-rules varieties of faerie blood may be added further ones suggested in Realms of Power: Faerie:
* Brownie-blooded are also short of statue, and look older than they actually are.
* The nymph-blooded (surely relatively common) have dark green hair, pale green skin, and leaf patterns over their body.
* Selkie-blooded or swanmaidens have webbed fingers or feathers in their hair.
* The children of troll-wives, of Ettin-blood, have craggy features, gray knobbly skin, a bulbous nose, and tiny eyes – and avoid direct sunlight.
All these characters often have other Virtues and Flaws that further accentuate their non-humanity, such as having a Small Frame or Giant Blood, bearing a Lesser or Greter Malediction (e.g. sunlight is painful), a Hunchback, Curse of Venus, Enchanted Music, Skinchanger, Greater Immunity to Drowning, and so on.
Some characters instead carry Magical Blood. They have physical abnormalities much like those of Strong Faerie Blood. Animal-blooded characters have wings, scales, gills, teeth, or claws. Those descended from magic humans have an "otherworldly appearance" related to Strength or Stamina (or, rarely, Presence). Those with the blood of a magic spirit or thing has an otherworldly appearance when using their powers – they eyes might turn black, they may rise a foot above the ground, their hair may be blown about by a wind no one else can feel, the skin can become dark with sort and smell like smoke, they may become semi-transparent like water, or so on.
Those with (Magical) Monstrous Blood may have similar physical deformities. The blood of magical animals results in similar, though non-functional, animal body parts. Magic human heritage may result in a dowdy appearance or sickly skin. And those with the blood of a magical thing have a detrimental Lesser Power, though this may not be immediately noticeable.
There are also those with Giant Blood, standing as high as 8 feet tall, head and shoulders above mortal men.
How common is such supernatural blood, how extreme are its visible features, and how accepted are such characters in society? Consider that merely being an Outsider, like a Jew, is considered a Major Flaw and indicates that you are shunned and often persecuted, your life and freedom occasionally being thrown in peril. Yet the above Virtues and Flaws do not carry such a Flaw, even as a recommendation. Realms of Power: Faerie suggests taking the Disfigured Flaw for characters that have such features and are not of strong faerie blood; it is reasonable to apply the same social treatment to those that do have strong faerie blood, too.
This would imply characters with obvious supernatural blood are not respected, and have cruel nicknames, but are largely accepted as part of mundane society. Which would in turn imply that they are not too rare. It would also suggest characters with strong fearie blood likely have only one distinguishing feature, and perhaps in a relatively minor form, rather than the whole appearance depicted in Realms of Power: Faerie and repeated above. Magical and monstrous bloodlines probably have similar levels of acceptance and non-human appearance.
The people in the streets of the city, therefore, might very well include non-humans not unlike D&D. Each such person would have only one distinguishing physical feature, not the whole suite, and while generally disrespected they would still be a part of society, not Outsiders.
Alternatively, you could consider those that bear an obvious supernatural blood to be Outsiders, requiring them to take the corresponding Flaw. This might be more suitable for characters that appear more inhuman, partaking in the above descriptions more fully. This goes beyond the rules, however, and the world they imply.
It perhaps should be noted that there are also people with angelic or demonic blood. Indeed, people with the Blood of the Nephilim might be not entirely rare, and may be added to the above list of the supernaturally blooded
However, those of Strong Angelic Heritage appear to be largely Mythic Companions and actual Nephilim (Divine giants), that the text implies are exceedingly rare. Similarly, those of Demonic Blood appear to be Devil Children, another type of Mythic Companion, and likewise rare.