I find it odd that so many classic rpg will just go with the default human and humanoid characters (especially humans, elves, sometimes kemonomimi or nekomimi characters, and other mostly humanoid or human-hybrid species). Okay; I know a lot of non-human races won't be very represented in the classic RPG, because they cannot or would not have the tendency to equip armor, weapons, and items the same way, if at all. Can a slime creature equip armor without absorbing and dissolving it? Perhaps not. Can a gryphon or harpie hold a sword in their talons? Again, likey not. Where are the recruitable bears, lamias, harpies, gryphons, and slimes?
I did once have a minor daydream about making a game where nonhuman characters and creatures could be just as recruitable as, say, Viktor and Flik in a Suikoden game. (Speaking of that, Suikoden games did have a small selection of recruitable non-humans, very small indeed; it might be interesting to have games where just as many of the characters if not more than the humanoids were specifically not humanoid.) To that end, the concepts relevant to including race as a primary controlling aspect of characters as well as classes must be handled first. This means, race defines the traits and characteristics which the potential classes must obey, and so forth down the line to characters themselves. It might be as simple as size, mass, and other physical properties.
A race definition must include a default growth curve (and maybe bonus growth, for variability) for each of the rpg system stats, and list of innate skills, to tweak the experience, leveling, and advancement for an individual of this species for each stat. This is to remove some of the stat-leveling from the classes themselves, and to keep it relevant to the race instance that a particular character is. It's to simulate the growth potential between a human and a dwarf, in a game where dwarves make excellent blacksmiths, craftsmen, and tinkerers; level one human and dwarf characters might be around the same level of prowess at crafting something in a forge, but for each character level of advancement, the dwarf might get a pronounced edge over the human when it comes to forging weapons, armor, trinkets, and such. -- A second example might be that avian species would possibly have flight as a capability that non-avian species just could not do, or at least unassisted.
Also, due to the inferred biological differences in a race's form, the race could or would possibly have its own unique equippable slot definitions. I will presume that most humanoid bipeds would share the basic same equipment slots for headgear, footwear, chest/torso armor, pants, and perhaps some hand or arm armor; but the difference becomes obvious when something like a harpie or a lamia is involved. Neither harpie nor lamia wears footwear; a harpie might be able to adapt some pants or skirts, but a lamia creature doesn't have the legs to wear pants, and a harpie would not be able to wear gloves or handwear due to its wings. Other non-human species might have just as unique equippable points on their bodies.
So to this point, we have custom stats ranges, skills, equippable slots, and ... what else? Well, above I wrote something along the lines of classes must obey the race definition; but on the flip side of things, a race should also allow or limit which classes a character of that race may be -- a harpie cannot be a very good archer, if at all, and a sentient bear-like species might be a natural-born mauler class of combatant, with their long, sharp claws.
Also, if we are in any way implementing a realism-like method of limiting party item-carry capacity, it might be handled from the race definition itself; or at least as a baseline, the individual character might have other strength-related bonuses which boost the carry capacity. An aforementioned sentient bear species, due to likely larger mass and musculature than most humanoids, have a quite larger carry capacity but not need as much proportionally-larger a set of armor than their humanoid fellows in the party. -- How it works is this: the party's carry capacity would be a computed total of each party member's individual carry limit, subtracting the weight of their armor, weapon, accessories, and items. (Item weight is one additional thing which would be required. It might even be a unique twist to an RPG, that size and mass affects carry capacity: a party can buy that fancy class-specific item, but if a member of said class does not have the carry-ability for its weight, the item is just extra weight in the party inventory. Very unique twist, indeed; one that would require the player to be very tactical about which things are equipped and carried.)
And it goes on like this; the main point of having a race or species for organizing characters and classes, is to remove some similar functionality from the characters and classes definitions themselves. In the long run, this will allow us to do even more interesting things in characters and classes, down the road.
... I feel all typed out, now....