There were already some suggestions about adding healing herbs, but i think it shouldn't be that simple. There should be special recipes for different potions, compresses or dyes. Moreover herbs should be rare, collected inly on certain terrains, or even time of the day, like dewy lady's mantle (the best would be season restrictions, but there are no seasons in this game). They should be prepared in different way for different uses (dried, boiled, brewed) and so on. Generally it should be very difficult and take a lot of time to make a healing potion or anything else.
I have herbalist friend and I help her on a historical fest here in Poland so i know something about this stuff. It was important knowledge in ancient times and it was so great than only chosen people had it, usually they were lonely women, because they had time to go around collect herbs and prepare them. Later they were called witches
---------------------------------------------------------------
Edit:
This will be a long post, but I wanted to put everything together because it's all connected. It took me a lot of time to think it up, I hope it won't be totally in vain...
First, about food decay there was already a discussion http://www.havenandhearth.com/forum/viewtopic.php?f=5&t=10728 so I'll add only my suggestions. I have whole cupboard of dead chickens and another of fresh meat, it's so stupid, so I think food (and herbs) should decay
PRESERVING:
Meat
- smoking - You can make simple smokehouse from branches and fir bough, but obviously it's small, has low quality (that lowers quality of food) and decays fast. Later you can build a solid smokehouse with bricks or clay.
Smoked meet have similar qualities as grilled, but takes more time to prepare, it lasts much longer but still decays after some time.
You can smoke fresh meat, whole fish, and sausages. - drying - Meat can be dried on drying frames after being smoked. It won't decay at all but it looses some of it's qualities (half?).
Vegetables and mushrooms - pickling or souring in jars. Need more thinking. (I love sour beetroot soup)
Cooked or baked food, eggs and milk can't be preserved. But for example bread lasts much longer than other foods.
Cheese and honey don't decay.
DECAYING:
I think food could decay in two phases:
First it's turning into old food - it looks the same, there's only "Old" or something like that added in the name and quality is lowered (halved). Eating old food can cause stomach's disease or even poisoning (see below).
Later it should turn in something uneatable (mess?). I don't think it should disappear, clean your mess after yourself
Old dead chickens and rabbits after butchering turn in to low quality bones and fur/feathers and old meat.
SICKNESSES:
Why? Because this is survival game and sicknesses are serious threat when trying to survive in wilderness. Did you see the movie "Into the wild"? If not, you should, I strongly recommend it for all H&H players, I won't spoil it.
Sickness is something you have to be prepared for, if not, it can be dangerous. When already sick you're better to depend for someone to help you get out of it, then try to do everything yourself (unfortunately you can use alts for this).
When falling sick you get certain amount of sickness points (separate for each kind of sickness). It should be based on your stats so it's very low when you are a newbie, but gets higher while you level up, so it won't get too easy to cure it later. When you drink herbal tee or use leeches it lowers the amount of sickness points, but it can also get higher. The strength of sickness' effect depends on sickness points (SP).
Cold
- there's a low possibility of catching cold all the time, but it gets higher if you stay in the water or a swamp (or cave/mine?) The longer you stay in it, the higher probability of caching cold
- it lowers your stats (depends on SP)
- you can cure it by sleeping for a long time, with herbs or leeches
- it will worsen if you walk around
- eating old food can cause it
- it slowly drains stamina (depends on SP) and you don't get any FEP bonuses
- you can cure it with herbs or leeches
- eating food can worsen it but you can drink tea and herbs, there might be also a safe dish - broth from fresh chicken meat and carrots
- low probability of poisoning when eating old food, there might be also different ways of getting poisoned - eating poisonous food, for example Royal Toadstool, or well poisoning by enemy.
- slowly loose SHP over time (depends on SP)
- cured with herbs or leeches
- it will worsen if you walk around
Healing wounds:
I think you shouldn't be able to use leeches for healing wounds. I haven't tried gauze yet so I don't have opinion on how it works.
Herbs for healing wounds yow brew and put in a jar, then apply it by putting a portion on a free spot in your equipment, or using it with gauze. When used without gauze works only when you sleep and disappears if you walk around. You can apply only one compress at time.
HERBS:
Most of herbs have to be brewed like tea, fresh fruits can't be mixed and brewed, but you can dry them and than mix with other herbs for brewing. Herbs works a certain time after drinking/eating/applying (about hour?), than you can use it again (similar to curiosities), you can drink/eat two times in a row but it won't have more medicinal effect, or it can have negative effect.
Single herb is very week, you can use them separately, but the more you mix together the better effect you get. Works similarly to FEP, but you first mix them, than brew and drink. Some herbs can be used only as ingredients mixed with other herbs.
First some herbs that are already in game:
Dandelion - mostly for stomach's disease and poisoning, ingredient for cold, fresh - weakly for wounds instead of compresses but it can't be mixed and brewed with other herbs.
Blueberry - for stomach's disease and poisoning
Stinging nettle - some modifications:
(for herbal use best collected during waxing moon)
- fresh you can change into nettle string (with water?)
- fresh mixed with honey (in a jar) - for cold and stomach's disease (can't be mixed)
- dried - for all sicknesses and wounds
Candleberry - for cold
Tangled Bramble - for stomach's disease and weakly for cold and wounds
Poppy flower - weakly for cold, overdosed will cause poisoning
Flax seeds - ingredient for stomach's disease
Mulberry leaves - ingredient for cold
Bark - ingredient for wounds
And some popular herbs that could be added:
Narrowleaf plantain - (Grasslands, Moor, Heath) for cold and wounds
Wormwood - (Forests, Grasslands, Moor) for stomach's disease, overdosed will cause poisoning, dyeing green, making absinthe
Elderberry - (Forests, flowers tn the afternoon only) You get flowers or fruits:
- flowers - for cold and wounds
- fruits - for stomach's disease and poisoning
Dwarf everlast - (Moor) stomach's disease and poisoning
Yarrow - (Grasslands, Moor) for stomach's disease and wounds and weakly for cold and poisoning
Coltsfoot - (Mudflats) cold
Camomile - (Grasslands, Moor) stomach's disease, weakly for wounds
Field horsetail - (everywhere except Swamps) for poisoning and wounds
Marsh horsetail - (Swamps) poisonous, looks the same as Field horsetail
Woad - (Moor) cold and poisoning
Blackthorn - (Broad-leaf forest, Moor, Heath) stomach's disease
Black Nightshade - (Forests, Grasslands, Heath, Moor) cold and stomach's disease but may cause poisoning
Birch leaf - ingredient for cold
These are of course only examples. I was choosing herbs that you can find growing naturally in Central Europe that's why I didn't mention for example peppermint.
Last four i added mostly as ingredients for dyes
DYEING:
Dying could be added if there were more cloths available, for example:
- woolen cloak and hood - protects from cold
- linen or silk dress for women - lowers agility but adds charisma
- linen bags for storing dried herbs
For crafting a dye you need hot water in a cauldron, a bucket and a dying ingredient. You make one liter of dye at once, but usually you'll need more (depends on cloth size)
Dyeing ingredients (only fresh):
Deep red - Carmine scales - (insects found on plants http://en.wikipedia.org/wiki/Perennial_knawel growing on on Moor)
Pale red - Tipton weed, Poppy flower
Yellow - Dandelion, Stinging nettle, Field horsetail, Birch leaf, Tipton weed, Blackthorn or Yarrow
Orange - mix yellow and pale red
Deep blue - Woad
Pale blue, violet, pink - Blueberry or Elderberry fruits
Green - Stinging nettle, Blackthorn or Birch leaf
Brown - Bark or birch bark, Blackthorn
Black - Bark+iron or Black Nightshade
To make linen cloths white you can hang them for a long time on a drying frame.
I haven't found anything about dying leather with natural dyes, so it either has to stay brown or you can use the same dyes but you get only dark warm colors (no yellow, blue, white, pink).
(I'm sorry for any grammar mistakes missing 'a's and so on, my English is far from being perfect, I used dictionary a lot.)