I would say to avoid mithril because it's become fairly generic and already exists in some other mods, but a metal that only the kitsune can refine might be a nice touch.
I know a tad about how kitsune are portrayed in mythology, but not anime.
1. Shape-shifting can be done with raw-modding, but it is overpowered (the creature is fully healed every time it changes shape). If that doesn't bother you, make a greeting interaction that can only target a single major race, giving that person an interaction that can transform a kitsune into their race. Add versions for each major race, and you have kitsune who blend in so long as the population is homogenous.
2. I'm curious what would happen if a playable race had the [MISCHEVOUS] tag. I recommend keeping each lever behind a forbidden door. And possibly a moat.
3. You cannot change the bodyplan with age (unless you do transformations, which have their own problems), but you can change the color pattern of a tissue using the same mechanism that turns hair gray. Make custom color patterns that correspond to having "numbered one", "numbered two", "numbered three", ... "numbered nine" tails and fiddle with the TLCM_NOUN to get something appropriately odd like "tails are numbered five" that advances with age. I can't think of a way to get one tail to use proper grammar in that system, but there might be some other approach that works just as well.
4. Bewitching, possessing, etc. probably require DFHack.
1, I'd be unlikely to go for shapeshifting, or try to make multiple variations. As much as I'd like to go with the traditional model of kitsune, I think that would be a bit.. out there on the overall complexity and power scale in DF. Seeing Immortal-D complain about cupronickel being overpowered, makes me fear what that would do. So... figure more long-lived anthropomorphic fox people with a single tail, rather than shapeshifting magic-using prankster fox demigod level spirits.
2. I... might actually pull that mischevous tag, just to see, if I added it.
3. I do think that may be overly complex?
4. Yeah.. trying to hold off on the innate magic side of things, as covered above, with the present DF model. Years ago I made fireball throwing, shapeshifting fox people for DF 2012 (files since lost) and... well, adventure mode was OP, world gen saw them basically own everything, and fortress mode was... interesting to watch?
I know you're modeling real-world values for Cupronickel. However, gameplay-wise, it is incredibly easy to make. Would it be possible to have the alloy require multiple bars and/or flux stone? A resource-intensive alternative to Steel would be ok, as that would prevent you from spamming it too early. On the subject of Mithril, that's fair. I would still like to see at least 1 other edge weapon weapon metal added, since all of the new stuff is for blunt & armor.
As to new races- I think Kobolds would make a good first step. They are part of vanilla DF, so developing them as a race shouldn't be considered going against the realism of DF. Masterwork has a good Kobold Fortress, you could look at that as a basis for developing Orichalcum Kobolds. I was never big on random animal-people being part of Fortress Mode, just not my style. Speaking personally, I would much rather see your efforts directed to more traditional / 'realistic' fantasy race like Kobold or Orc.
It already does require multiple bars - 3 copper and 1 nickel bar. (75/25) Flux stone is not needed in real life, as they are metals which naturally alloy well.. Realistically speaking, the only reason cupronickel wasn't more common in histortical use and such is because most nickel ore is really hard to visually distinguish. Garneterite and baitong are exceptions, but one is rare to see surface-reaching veins of, and the other is only found in a particular area of china and was declared something to be kept secret for over a thousand years save for select smiths.
So.. yeah. As for kobold realism, I.. honestly see kitsune as more realistic than kobolds, given mammalian intelligence. (I wouldn't be planning on going all out there on the magic side of things.) As for traditional fantasy race, fox spirits and fox people have stories in every culture on the planet. They might not have a large presence in the more modern Tolkien - and - DnD inspired fantasy, but as far as fantasy stories are concerned, they are fairly traditional.
That stated, would be quite willing to go for Kobold first as a lower tech / higher challenge race type thing.
