Brand new player here... I came looking for mods because there were two glaring problems in the vanilla production chain - missing millers for grain (no-one eats raw grains unmilled) and missing tanneries for leather (you can't do anything with untanned leather, let alone make clothes out of it

). I'm incredulous that this wasn't part of the vanilla game since this is so basic and an expected part of production chains in about all other building games I've played. I'm very pleased to see modders have addressed the milling problem and provided good consistency for milling flour/baking across their mods, but the approach to tanneries is rather inconsistent; or perhaps they aren't very interesting to modders.
My main problem is that tailoring would never use a raw animal hide without it being scraped clean of fat and flesh and hair, which is what the tanning process does. If there is going to be any reality in the production chains (which has been happily explored or taken to extremes in other areas, with a plethora of baking products, milk and cheese, even coke and charcoal) then tailors should
never take raw hides from hunters/livestock farmers. I can see the difficulty of changing it now, as unless there's a possibility for a simple over-ride you either have to change the raw product resource for every possible modded hunter model and animal type that might drop "hides", or stop tailors from picking up "raw" leather, which means altering all possible tailors out there to accept all possible cured leather resource types. But since that work has been done already with the millers and flour from grains, I am surprised that there hasn't been more effort at a consistent approach for leather.
Right now I see maybe three mods (apart from CC which I don't want to get into right now) that add tanneries, but each does it in a different way, so they aren't really mixable, because of the way the raw material leather (hides) and the tannery output (leather/cured leather) are used or inconsistently coded. I noticed RedKetchup in one post somewhere suggested he wouldn't do a tannery because DarkBibou's old Tree of Life mod did it already, but that has a lot of drawbacks AFAICT, especially the lack of recent updates and the fact that you have to have roaming cows and chickens in your forest for hunting, which is nonsense for some of us. The good part of that is that it seems to insert before any leather usage by any other business, so there are no changes needed for vanilla-based tailor requirements, for instance. EMB's Village production has a nice solution producing cured leather, but the leatherworking using that seems to be restricted only to producing unusable goods for export (pouches and saddles), not inserting in the tailoring chain. Other mods have a shoemaker, but again that's more extra unusable goods instead of it being part of the critical tailoring chain.
I also want to comment on the tannery process/building itself: tanneries are deeply unpleasant buildings to live near. Throughout history the basic production method was to soak your raw hides in urine and even dung, since uric acid was the cheapest and most easily available active ingredient in the process before a formal chemical industry. I don't believe salt was ever part of the medieval method as it was an expensive food import from the very few places that could mine it (or create salt pans near the coast), almost as precious as spice. But that does mean that in-game any tannery should have a
very significant unhappiness factor around it, like a mine but probably more so. I've been to Fez, the almost unchanged medieval town in Morocco, which still has original working open tannery pits in the centre for their famous leather export industry, and even now in the modern era the stench is almost unbearable, so using human and animal urine in the past in Moroccan heat must have been... challenging for the locals.

Here's a good guide to them:
http://www.onlyadayaway.com/visiting-the-fez-tanneries/ - although I'm not suggesting a change to the models from the current covered production buildings.
I'm putting these thoughts out there in the hope that some of the still active modders might consider them in updates to their tannery chains, or by adding more realistic tailors that don't take smelly, bloody, hair-covered raw hides to make clothes from.

The production chain could also use fertiliser (as dung dropped by animals) rather than salt, or just assume that the source is the population themselves, which would be historically accurate, and would allow an immediate industry startup rather than having to wait for livestock (or build salt).
It would also be nice to see some brainstorming to make this more consistent across mods, although I'm aware that compatability with CC is likely to be the biggest argument most will have against any change. Since this has never been discussed here AFAICT, I'm interested to see the feedback.