DnD Head Canons

[PairO'Dice Lost]

You do realize about half of your creation myth is straight from Golarion, if you squint a bit. Goblins there are basically just bipedal demonic dogs. Gnomes are not foxes, but the next best mythical thing, fey creatures. Elves are aliens (that started on Golarion but came back), dwarves were forged by a god. Your orcs are spot-on as well, with them being proto-humanoids at the start of recorded history.

Technically, half of Golarion's creation myth would be straight from my setting, because I did my setting workup back in 2006ish before Golarion was a thing. On the Campaign Builder's Guild, if anyone remembers that site before they migrated and lost all the old material. Ah, the good ol' days for homebrew.

But in general, the "different races have very different origins instead of being humans in different hats" thing isn't all that novel. Elves, dwarves, and gnomes being somehow special (in-game because they have their own racial pantheons doing their own thing, out of game because they have more of a fantasy pedigree than halflings and PC-side orcs and goblins and such) has been done for a while, and in fact "elves come from a different planet" was done in Forgotten Realms and "dwarves were forged by a god" was done in Dragonlance long before most spinoff or homebrew settings existed. Meanwhile, the "different evolutionary line" explanation came up a lot when orcs and kobolds looked more obviously pig-like and dog-like, respectively, and lots of the early monstrous humanoids were basically half-animal people (lizardfolk and hyena-people [gnolls] and fish-people [sahuagin and kuo-toa] and so forth), and one of the reason OSR products are full of frog people and snake people is because they're going for the same "humanoid monsters are more primordial/prototypical creatures than the modern races" angle.

What would be novel is headcanonizing an explicit origin for humans that doesn't just put them as the "mix of all the other races"/"collaboration between gods" race, since everyone likes to play around with the various demihumans but nearly always leave humans alone.

EDIT: Actually, along those lines...

-Metalic Dragons are essentially the same species as their Chromatic counterparts. They just mutate depending on the Alignmental Energy they channel. A good Red Dragon will eventually turn into a Gold Dragon, and an Evil Bronze will turn Blue.

So, humans and dragons both have four interesting things in common:

1) They can breed with basically anything. Part-human planetouched are everywhere (compared to part-other-race planetouched, which are more rare and only come in specific combinations) and near-humans like rilkan and illumians are plentiful, and half-[elves/orcs/dwarves/etc.] are a thing while e.g. orc/dwarves and gnome/elves aren't. Half-dragon and draconic creatures are everywhere, and there are even true-breeding part-dragon offshoots like chimerae and dragon turtles.

2) They come in different colors. Dragons obviously have their very colorful metallic, chromatic, and gem dragon families plus a bunch of one-offs (shadow dragons, planar dragons, etc.) in a rainbow of colors, but humans are also pretty colorful in that they're nearly always the only race in a setting to come in a wide variety of skin tones for a single race; even elves tend to only come in pale/brown/black for high/wood/drow elves, with any other subraces fitting in those general categories.

3) They're disruptive to the status quo. Most settings have some backstory element along the lines of "elves and dwarves and giants and other really old and really-long lived races were chilling for a few thousand years, and then bam, humans came along and in less than a few centuries they're everywhere and running everything," and out of all the monsters out there, dragons are the most likely to just swoop in out of nowhere, burn down a few towns, and take over the area.

4) They're highly changeable and adaptable. All metallic and several other kinds of dragons have some sort of innate shapechanging capability, and all true dragons have distinct age categories that grant them different capabilities over time, while humans are generally the most adaptable PCs (in AD&D they could dual-class instead of multiclass and had no class or level restrictions, in 3e and later they have generalist ability score boosts and fewer or no multiclassing restrictions)

What other race can breed with anything, comes in a bunch of colors, disrupt everything they come in contact with, and are known for their mutations?

Slaad.

Positing that, like slaad, humans and dragons are creatures of innate chaos would put an interesting twist on a setting. It explains why humans burst onto the scene and took charge from the other humanoid races after they'd been chugging along in an orderly fashion for millennia, why it's always the humans that are setting up crazy-powerful empires that are constantly rising and falling and having major side effects on the setting, why (in earlier editions) humans work so differently from the other races mechanically and flavor-wise, and so on.

It would also give you a good opportunity to flip the usual implicit "Law is better than Chaos" assumption a bunch of people have. Make Paladins of Freedom the default paladin and all the good theocracies CG instead of LG! Toss the Law-leaning monarchies and empires with their feudalism and massive armies and such and fill the continent with Chaos-leaning city-states and federations with bands of adventurers privateers instead of standing armies! Paint the archons and devils as the truly alien and inscrutable beings that they are and paint the eladrin and demons as having more relatable and human-like motivations! Instead of emphasizing the Chaotic-ish weirdness and uniqueness of aberrations, emphasize the unnatural Lawfulness of illithid/elder brain hive minds and thrall-keeping aboleths and beholders!

And sure, there are lawful dragons and humans and they appear to average out to neutrality on a race-wide level, but everyone knows they're really all a bit chaotic where it counts. Even the naturally-Chaotic slaad churn out naturally-Lawful gormeels every so often.