So here's my analysis, circa 1.900. Feedback is very welcome. In the end I wound up changing exactly a third of the Japanese mythos to make them actually attractive, while two thirds I thought were already fine. If you disagree with me on specific points, please do let me know in the followup thread.
To save room, I suggest not posting about things you already agree with me on, since the list is long; just points of contention will do!
Amaterasu:
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-Enenra: Can only be seen by the pure of heart, so is unable to interact with bandits: it can neither deal damage to nor receive damage from bandits. Worth 2x the normal number of points on death.
--Seems useful enough, although only conditionally so. But that's the nature of some of this sort of thing, you know?
- Ushi-Oni: Protectors in the form of an Ox-Demon. Thought to drive away evil spirits, these have a large bonus against mythological creatures. They are also immune to ranged attack, and refuse to move. Worth 4x the normal number of points on death.
--Here again, this is a great "react to a problem" sort of ability.
- Okuri-Inu: Some wolves are known for following people to protect them. Any allied human units who are within 2 tiles of the Okuri-Inu are invulnerable to attack from enemies. Worth 5x the normal number of points on death.
--Here again, this is a great "react to a problem" sort of ability.
Izanagi:
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-Hihi: A baboon monster that is invincible while in bamboo forests. Transfers this attribute to any enemies that it attacks. Worth 3x the normal number of points on death.
--Okay, yeah, I can see how that increases chaos in a way that is not helpful.
--So, changing it to this, which is a lot more risk/reward: A baboon monster that is invincible while in bamboo forests. Transfers this attribute to any enemies that it attacks, while wiping out all the other bonuses the unit may have. Worth 3x the normal number of points on death.
-Gagoze: A lazy priest-servant turned demon-ghost who likes to kill children. No barracks, archery range, or siege workshop can produce units while he's within range of 3. Worth 2x the normal number of points on death.
--Okay, yeah; again this is helping the Japanese, who are probably already too powerful anyway when this would be played. It needs to be the other way.
--So, changing it to this: A lazy priest-servant turned demon-ghost who likes to kill children. No Japanese barracks, archery range, or siege workshop will produce units while he lives. Worth 2x the normal number of points on death.
-Keukegen: Cute and cuddly-looking, he nonetheless is a big cause of disease. You don't want to find one of these in your house. The turn after he is placed, the next woe strikes, ending the current woe. Worth 8x the normal number of points on death.
--This is super useful in the right context. If you have an ongoing woe that you really hate, this is a way to get rid of it, just with the cost of having another woe hit you sooner than it otherwise would. Circumstantial, yes, but still flexibile and cool.
Raijin:
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-Baku: Devours dreams and nightmares. Each turn, a single random non-god unit with bonuses (enemy or allied) within range of 3 will have all its bonuses wiped out. In place of those bonuses, a single Nightmare bonus will be applied to that unit: this makes them invulnerable to attack, but also inverts their auto-heal. Worth 3x the normal number of points on death.
-Yeah... okay, that is not good if you are playing it. Cool if the enemies do, but even then it's kind of random.
--Altered it to this: Devours dreams and nightmares. The turn after he is placed, the current and next woes will be replaced with completely new woes. Then, each turn a single random non-god unit with bonuses (enemy or allied) within range of 3 will have all its bonuses wiped out. In place of those bonuses, a single Nightmare bonus will be applied to that unit: this makes them invulnerable to attack, but also inverts their auto-heal. Worth 3x the normal number of points on death.
-Ningyo: A fisherman catching one of these was believed to cause storms and misfortune. These have a very low attack power, but will insta-kill most humans who attack them. Worth 2x the normal number of points on death.
--Seems very useful to me!
-Akateko: Literally translated as "the red-handed child," the Akateko is a tree with a bright red infant's hand dangling from it. Freaky, right? Non-god units that attack it feel the same way, and die of fright. The tree is unable to move, and has almost no health, but kills even those enemies who attack it from range.
--Even more useful!
Suijinsama:
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-Daidarabotchi: Giant responsible for creating most of the lakes and ponds in Japan. Every tile he steps on becomes a lake unless it is a building. Worth 5x the normal number of points on death.
--Seems almost exploitatively easy, as you can wall off bandits from part of the map, but the Japanese can still reach them. It's not an exploit thanks to Suijinsama's passive, but it's a good ability I think.
-Nure-Onna: Every human unit within range 3 is struck dead at the start of each turn when she is on a lake or bridged lake. Worth 2x the normal number of points on death.
--See, I could understand why you might think she is randomly negative, but I think she's actually really useful. She kills a lot of humans and gets points for them, under certain circumstances. The fact that she kills humans from her own faction is actually USEFUL if you want to farm some points, because odds are your faction is plenty strong without those humans anyhow.
-Kawauso: River otters with a very low speed and attack. For every one of these otters on the map, all units of all factions get a 2x attack bonus while on a lake, bridged lake, or marsh. Worth 2x the normal number of points on death.
--Yeah, very unpleasantly chaos-inducing. Either pointless or detrimental.
--So, this is way more fun: River otters with a very low speed and attack. While at least one of these otters are on the map, no units have the water-crosser ability (though those with flying can still cross water). Worth 2x the normal number of points on death.
Kagu-tsuchi:
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-Basan: A fire-breathing mystical chicken, Basan is fully healed when crossing bamboo forests. Worth 6x the normal number of points on death.
--This guy is awesome, I think.
-Nue: According to legend, can metamorphose into a black cloud and fly. On Luminith this is not strictly true: instead the Nue can run incredibly far across the terrain with its massive amount of AP; but its attacks cost a similarly massive amount of AP, so it can only attack twice despite being able to get anywhere on the battlefield in practically one turn. Worth 5x the normal number of points on death.
--Yeah, okay, that's pretty much something I'd never place.
--So this is much more interesting and useful: According to legend, can metamorphose into a black cloud and fly. On Luminith this is not strictly true: instead the Nue can run incredibly far across the terrain with its massive amount of AP; but its attacks cost a similarly massive amount of AP, so it can only attack twice despite being able to get anywhere on the battlefield in practically one turn. It is only able to attack units with at least three directly-held bonuses. Worth 5x the normal number of points on death.
-Hou-ou: The arrival of these Phoenix-like birds mark the birth of a virtuous ruler. The turn after they are placed, a random human unit from any faction on the map when they are gets a "strong leader" bonus, which gives that human the ability to grant, per turn, any one allied human unit within 5 of them a permanent 15x attack boost. Worth 8x the normal number of points on death.
--Okay, originally this was too few points, but that was already adjusted. More to the point, having this be any-faction (maybe even bandits) was a bad idea. This way (see below) you know it will be one of your guys, and so you can use it to turn the tide.
--So it's just for its faction now: The arrival of these Phoenix-like birds mark the birth of a virtuous ruler. The turn after they are placed, a random human unit from any faction on the map when they are gets a "strong leader" bonus, which gives that human the ability to grant, per turn, any one allied human unit within 5 of them a permanent 15x attack boost. Worth 8x the normal number of points on death.
Uke Mochi:
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-Kitsune: A nine-tailed fox that is very old and wise. Many people give kitsune offerings because of their supposed power and influence, almost as if they were a deity. Every turn, each enemy human unit within range 4 generates the equivalent of 1/10th of a soul imprint for the Kitsune's faction.
-Super useful!
-Tanuki: A raccoon-dog that is excellent at disguising itself to make people look stupid. Any human units that come within range 2 of it get struck with human vanity. Worth 2x the normal number of points on death.
--Okay, yeah, that's not as useful as I'd hoped. I figured it was a good way to disable a big battlefield, including the Japanese, which are often dangerous. But let's make this a way to help deal with monster caves instead.
--So now it's this: A raccoon-dog that is excellent at disguising itself to make people look stupid. Any enemy mythological creatures that come within range 2 of it get struck with vanity. Worth 2x the normal number of points on death.
-Kodama: Spirits that normally live in certain trees in the inner reaches of mountains. Killing a Kodama via direct attack brings misfortune in the form of the loss of ten human soul imprints from the stockpile of the Kodama's faction. Worth 12x the normal number of points on death.
--What happened here is that I got so caught up in being thematic that I forgot to make it useful. Oops.
--So now it's: Spirits that normally live in certain trees in the inner reaches of mountains. Killing a Kodama via direct attack brings misfortune in the form of the loss of the attacking unit losing all bonuses it may have. Worth 6x the normal number of points on death.
Fujin:
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-Tengu: Troublemaker bird of prey. Doesn't attack bandits, and cannot be attacked by bandits. Any non-god unit it attacks without killing becomes a bandit, and immediately grants points as if that unit had died.
--This one is borderline, but I'm inclined to keep it. My original thought was that it plays well with some of the Greek and Norse abilities. That said, even here it is useful as a source of points, because you can kill a single enemy unit twice.
-Zorigami: Immune to ranged attack. Does no damage. Wanders at random at speed 1. All human units from any faction that come within range 2 of it get their AP permanently quadrupled. Worth 4x the normal number of points on death.
--Okay, yeah. That is way too dangerous there, for no real reward that is commensurate. So let's make this have some risk/reward going on:
--Now it's: Immune to ranged attack. Does no damage. Wanders at random at speed 1. All human units from any faction that come within range 2 of it get their AP permanently quadrupled AND their score-on-death worth quadrupled. Worth 4x the normal number of points on death.
-Bakezori: An old sandal that was mistreated, and now runs through the house chanting "kararin, kororin, kankororin!" All enemy units within sight range of this annoying creature will consider it the very highest priority to attack if they can do so. Very high health. Worth 2x the normal number of points on death.
--The ability to distract the enemy for a while? Super useful!
Ebisu:
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-Ittan-Momen: A roll of cotton cloth that attacks people by wrapping them up. Any non-god unit it attacks becomes permanently unable to move (but still able to counterattack). Worth 4x the normal number of points on death.
--Permanently disabling enemy units from moving? Yes please!
-Mujina: Mystical badger with mid-power attacks. Particularly known for shapeshifting and deceiving humans. Any humans from any faction that come within range 2 of it become fearful of faceless ghosts in enemy buildings; they cease being able to walk on or attack enemy buildings, and become unable to move if they are already on an enemy building. Worth 2x the normal number of points on death.
--Protection for buildings? Again: yes please!
-Nuppeppo: A passive, genderless, blobby creature that does not move and deals no damage. All human units from any faction that come within range 3 of it become paralyzed by its extreme smell, which is almost like rotting flesh. Worth 2x the normal number of points on death.
--Disabling all human units in an area? Strikes me as quite useful even though it's getting your guys, too. Sometimes -- especially with the Japanese -- that's simply necessary.