Since hope springs eternal, I figure that if I'm going to ask for advice on how to finish up these remaining races on my list, I may as well share what I know about them and what ideas I actually have for them.
Just an aside, reading over the old entries for the Hengeyokai race reminded me that it traditionally includes a rabbit subrace. I'm not aware of any specific rabbit yokai in Japanese mythology, but I would be interested in learning if that is the case or if there's otherwise a ground for a rabbit-based race in 5e.
Also... once I actually have all of these races done and balanced, I will doubtlessly look towards more. Whilst there's some D&D/PF monstrous races that might work - Centaur, Thriae, Girtablilu, Sahuagin - I was curious; anyone think there'd be interest in video game-based races?
Pterran: An official race from 2nd edition, Pterrans were first introduced in the Revised Campaign Setting boxset for Dark Sun as essentially Athas' replacement for lizardfolk; closer to pterodactyl or dragon-men in physique, they lost their wings eons ago. They were later updated for 3.5 in Dragon Magazine #319, as part of its wholesale Athas update for 3rd edition. Nothing really fancy, their thing was being strong and resilient (natural AC bonus; they were base AC 8 in 2e and got +5 natural AC in 3.5) with powerful claws. Ironically, they went from being a +1 Str and +1 Wis race to being a +2 Str+Con and -2 Wis race with the edition shift, despite their lore portraying them as a naturally druid/earth cleric-revering society. This is a race I'm struggling with because, frankly, they're kind of bland; the strength bonus, the natural armor and the claws are pretty much all they've got going for them. I think I need to discuss them before I can tackle them.
Spellscale: Introduced in 3.5's Races of the Dragon, Spellscales were a race of draconic humanoids most defined for looking like scaly elves. With a strongly chaotic mindset, they were most categorized for being natural sorcerers, which manifested in their Charisma bonus and favored class. Their most unique attribute was their Blood Quickening, where each day they could use meditation to align themselves with one of the draconic deities and gain certain boosts based on that deity, a mess of an ability which I'm doubtful can be used directly. What I'm thinking of doing is giving them a subrace based on Draconic Ancestry, with each type of dragon giving them different racial features and spell-like abilities. Does this sound viable to anyone?
Bladeling: First appearing in the original Planescape monstrous compendiums, Bladelings are an outsider race resembling tieflings covered in bristling metallic barbs and spikes, native to the plane of Acheron. Despite their xenophobia, they did receive PC writeups in both 3rd edition (MM2) and 4th edition (MotP). 3.5 gave them damage reduction against slashing, cold and fire resistance, immunity to acid and rust attacks, the standard ousider resistances, and a daily "razor storm" attack. 4e specified them as Dex and Wis boosts, gave them acid resistance, and Razor Storm as an encounter power. This race is, I think, a fairly straightforward conversion. I don't know if I can get away with even a natural AC boost, nevermind slashing resistance, to represent their spikes, but acid resistance is definitely fair game. Razor Storm should be convertable, as well. Perhaps a racial trait that lets them automatically inflict damage on foes who grapple them, and/or whilst grappling?
Fey'ri: Introduced in 3.5's Races of Faerun, the Fey'ri are essentially elfin tieflings. They naturally have wings, though they can use Alter Self to shapeshift out of that form, but they also have 4 from a small list of various powers, including fire resistance, saves vs. electricity or poison, 1/day spell like abilities including charm person, suggestion and dimension door, and damage reduction. Lore-wise, they were almost universally elf/succubus crossbreeds, and I think their powers reflected that. I'm not really sure how to handle these ones in 5e... should I give them some of the same flexibility by building on the sub-race mechanics, or should I try and stick more clearly to the elf/succubus fusion aspect?
Rogue Modron: One of the most beloved of Planescape races, if I remember correctly, the Rogue Modron is a Quadrone who has succumbed to the subtle lures of chaos, developing an independent personality of its own and deviating from the mindless automony of its race. I... actually don't remember any abilities they had, or even where they appeared, so I could use help from people who know them better.
Hamadryad: Made a playable race in the 4e book "Heroes of the Feywild", the Hamadryad is essentially a "upper tier" dryad from AD&D, if my memory serves me, who champions a whole forest rather than a specific tree, which gives her an increased amount of freedom to operate. I don't actually remember that much about the race and it mostly serves me as a "let's try and add more non-elf faerie PC options!" excuse.
Satyr: This has pretty much the same deal as the Hamadryad. Both of them really need a lot of discussion, because I'm sure that there's the kernels of interesting races in there somewhere, but I need a proper think-tank to bring them out to their full potential.
Huldra: This one was another "I think 5e needs more faeries", but it owes itself to a result of my confusing the Uldra (a sort of icy fae-gnome or frost elemental halfling) from 3.5's Frostburn with Pathfinder's actual Huldra, which is based on... I think I think Swedish mythology? It's a mountain nymph with superhuman strength and beauty, but it has either a fox's tail (or maybe a deer's? Don't remember all the details off the top of my head), or a back that looks like a rotting/hollow log, so it tries to avoid letting people see it from behind. There's definitely potential there, it just needs work.
Dust Genasi: Despite what you might think, this Genasi has actually appeared before; full-fledged Dust (the quasielemental plane of Earth and Negative Energy, if memory serves) Genasi appeared in Dragon Magazine #297 for 3.5, and Sand Genasi were part of the Athasian Genasi article in 4e's Dragon Magazine #396. I don't remember their abilities off the top of my head, but I'd say that some kind of stealthy assassin "basis" for the race makes sense, yes? Dust represents the end of all things, the total destruction of everything, dissolution and entropy. Maybe Necrotic Resistance and some kind of "sandform" can be swung?
Rain Genasi: Inspired exclusively by the Rain "demielemental" sphere for Clerics in Dark Sun, I envision this as mostly a healer type Genasi, though with a dark edge; rain brings life, but also brings deadly floods, and can be fickle in when and where to show itself. +1 Wisdom seems most fitting for the stat bonus, and perhaps Cure Wounds or whatever the healing cantrip is as an SLA, but I can't really figure out how else to support this theme.
Magma Genasi: This one is surprisingly challenging for me given that it shares the same lineage as the Dust Genasi. Magma is basically earth plus fire, but I don't really know how to give it more of an "oomph" than that. Really would like help figuring this one out.
Blight Genasi: This one, however, is fairly solid, at least in terms of a basis; it's the "radiation" or "pollution" elemental, the genasi born where the land and water are made sick and foul, with touchings upon the general concept of the Abyssal Genasi in 4th edition. Not sure what stat boost to give it - maybe just a further +1 Constitution? Poison Resistance is a must, maybe Necrotic or Acid resistance too? Able to survive longer on less supplies, and/or to consume things that would be too diseased or foul for humans to consume? Poison Spray as a racial spell like ability, obviously. Not sure if there's any other spells that further that theme. Should be resistant, or even outright immune, to disease. I can't imagine them tasting very nice... maybe a racial ability building on that? Say, inflict a certain amount of Poison damage on someone who uses a bite attack on them, and if swallowed whole, the creature must make a Constitution check each round to avoid spitting them back out again?
Smoke Genasi: Although this did appear in the same Paraelemental Genasi dragon article as the Magma and Dust Genasi, I'm struggling with ideas to make this interesting.
Tanuki: Perhaps the most recognizable of the hengeyokai outside of the kitsune, the tanuki is a cheerful, if somewhat dimwitted, aspiring trickster. Like the kitsune, it's usually stated to be very adept at shapeshifting, though I think it's more famous for turning into objects than people? Although it's never appeared in any fashion in D&D, it has appeared as a monster in Pathfinder. I'm thinking +2 Constitution, for the race's association with loving to drink sake, and +1 Charisma, because they are supposed to be tricksters, if less skilled at it than kitsunes. Abilities are throwing me for a huge loop... I was wondering if maybe some sort of "polymorph self into a tool or object" power would be too overpowered? Maybe an illusion-based spell-like ability is fitting as well. Beyond that, I got nothing, other than the thought that as either a racial ability or a feat-unlocked power it could be capable of flight? That isn't something Nintendo made up for its Tanuki Leaf/Suit powerups, that's actually something that tanuki can do in some myths. Hmm... I think I also remembered that Tanuki are associated with good luck (which is at least partially what those huge testicles symbolise), so maybe the Halfling's Lucky feature can be borrowed/stolen/improved upon?
Bakeneko: The root of the various shapeshifting cat yokai, it's hard to really specify the difference between it, the Nekomata and the Kasha. The bakeneko seems to be the general "base point" between the nekomata's focus as a shapeshifting succubus-type predator with necromantic powers and the fire-wielding powers of the Kasha, only nekomatas are universally evil and bakenekos can be good or evil... maybe focusing on the more feline succubus elements for this one might be a good idea?
Mujina: This is a more obscure Japanese hengeyokai, due in no small part to confusion even in the lands of its birth with the tanuki. This is a shapeshifting badger who... well, I think it mostly just does minor trickster stuff. It has actually appeared in D&D before, sort of, in that the Hengeyokai race (3e's Oriental Adventures, 4e's Ecology of the Hengeyokai in Dragon Magazine #404) has always included a Badger option. Its main bonus was that it gained a Burrow speed in hybrid/animal form but was slower in that form than in human form.
Itachi: Let's be honest, who here actually knew that Japan's hengeyokai included shapeshifting weasels? Most people are probably more familiar with the kamaitachi ("sickle-weasel"), a weasel yokai that moves at incredible speeds and likes to cut people in hit 'n' run attacks, than with the itachi proper. I'm not much of an expert on weasels, and so I'm not really sure what to give them; increased movement speed and a bonus to Dexterity (Acrobatics) checks seems logical, but otherwise...
Kawauso: This little charmer is one of the more forgotten hengeyokai; it's an otter that's learned how to assume a human form, and whilst normally depicted as a cute and playful little mischief-maker whose greatest crime is normally trying to buy, beg or steal liquor to get drunk, it does have the odd tale as a sinister being, such as one story of a female kawauso who used her human form to seduce men, only to eat them after she was bored with them. Speed swim and increased capacity to hold breath are definitely on the menu, but as for what else...
Oni: The Japanese Ogre is actually a little like the modern day D&D orc; most stories focus on its nature as a brutal monster, but there are stories of oni who redeem themselves, or at least are willing to fight even worse monsters. I think. This one maybe I should wait to tackle until we see the Firbolg and thus get the official skinny on how 5e is going to handle all Large races, but even beyond that I'm having trouble figuring out; Oni are big on strength but they're also magical creatures, if memory serves. Strength and Charisma as the ability score modifiers, maybe, and I'm sure there's some grounds for spell-like racial abilities.
Tengu: The Japanese birdman, I'm honestly not sure there's even a point to this as its own race between the Aarakocra and the Kenku. What do folks think?
