If a plane has some inspiration from something from our world I'll try to include creatures and races known or associated with the source, such as zombies (as ghouls) in my Deshub set that has some medieval Middle East inspiration. When filling planes not drawn from any iconic source I tend to grab at either what feels right based on the civilization or is just familiar to Magic players.

Are there any races or monsters you tend to avoid or always seek to include? I'm not keen on merfolk on land, so unless my plane has numerous, large bodies of water, I tend to skip them. I also forget faeries because they're associated with flying rather than any actual aversion. Though, acknowledging that, a few faeries filling in for slots usually given to birds in blue is something I could probably do. Oddly I've used Aven despite being associated with flying.

Sub-races that can be classified as a n existing race are usually pretty helpful, like Hobgoblins under Goblins, or Drow under Elf. Magic generally has a way to classify various classical races that aren't existing in MTG (i.e. Tieflings are imps or devils, etc.), but I'm usually not averse to adding new types if the source material calls for it, or if there's a good reason for them to exist on the plane.

Most of my inspiration is real-world mythology, so I try to use animals that are iconic to the region or setting. For example, in Xianlu, I chose Monkey and Turtle as the two key races because both are fairly prevalent in fiction. Bear (Panda) erred too close to Pandaren for me to want to use them, and the other members of the Chinese Zodiac aren't really prevalent or anthropomorphized enough in Chinese fiction for me to consider them. Aven also appear because birds in various forms are staples of Chinese myth.

I'd love to stop using Human as the default race, but too often any names that don't include clues towards a creature's race are filled in as "human" creatures. Very few planes feel "right" without humans or human-like beings as a race.

I'm generally tired of human as the default race in high-fantasy settings as well. Finding art for every card would maybe suggest that the speaker isn't a human? Unfortunately, many people need humans specifically to identify with. Such was considered one of the faults of Lorwyn-Shadowmoor.

Mal, what's your limitations of human-like beings as races? I would see elves and dwarves as overly humanesque in terms of appearance, but I'll give vedalken a pass, their features feeling slightly alien. How about orcs?

Generally, I consider a race to be "human-like" if they're the default for the setting, can exist in most roles/colors, and aren't necessarily tied to a mechanical theme. That's why I'm okay with Innistrad - instead of humans being the -default- race of many races, they're the origin race (with every other tribe represented in Innistrad just a various form of human - ghost human, undead human, furry human, pale human). I'm okay with human-like beings as races as long as they have a relatively cohesive mechanical or color identity to them.

Dwarves in Kaladesh, for example, didn't do much to differentiate themselves from humans. Aerial Responder, Fairgrounds Warden, and Consul's Shieldguard, for example, didn't need to be dwarves. If all of the dwarves were similar to Depala, Pilot Exemplar or Gearshift Ace, that would be less of an issue.

Conversely, if you had a setting that had Elves in all colors, with most of them eschewing their green, mana-producing, tree-hugging, swarming ways, you're probably just better off using humans as the default race. You're not necessarily impressing anyone with that aspect of your setting unless you do something meaningful with it. I feel the same with Orcs - too often they're easily replaced by the more MTG-default Goblins or Ogres, depending on the size. They don't really do much to separate themselves from those races. Orcs in Khans of Tarkir only felt like they were included because Wizards needed some alternate races to spice up some of their designs and world, not because they had any want for them mechanically. The same with Aetherborn - they look cool, but for the most part, they do stuff that can be covered by humans or vampires (if they existed in Kaladesh) in the setting.

If based on some story (e.g. "Ziveruskex and Strixan" story that I play a GURPS game), they will be mostly based on that (changed as necessary to fit the game effects), and will add additional creature types if it seems right to do so, but usually the existing creature types can be used (Elf, Bird, etc), mostly how Magic: the Gathering is already doing. If it matches an already existing card, then that card will just be used instead.

A set does not necessarily need humans, although it would probably be common. However, neither Human nor any other will necessarily need to be "default"; it can be selected individually for each card. They can then be adjusted as needed if it is wanted to work specific kinds of game effects that care about creature types, or if it would fit the story better, or if the artist had drawn something or is better to draw something else. (These can affect things other than sutypes too, though.)