This thing could be new state for player.

-Can get caught by being in really cold area with almost no clothes, be really dirty or medicine overdose.

Level 1 illness:
-This level of illness could contain sneezing, rarely coughing.
-Easilly healed (by any type of healing drugs (pills and etc.)

Level 2 illness:
-Low level of high tempature, coughing.
-Healed as level 1.

Level 3 illness:
-High tempature, big amount of coughing.
-Not possible to heal illness of this level that easily as the lvl. 1 and 2. (so you can not heal by normal pills)

Level 4 illness:
-Low dizziness, faster draining of fatigue, rarely vomiting, high tempature.
-Not possible to heal with normal pills and with regular medicine.

Level 5 illness:
-Dizziness, faster draining of fatigue (as in lvl. 4), vomiting, rarely hardly walking (so you are walking slower, sometimes falling down to the ground) higher tempature.
-Cannot be healed with normal pills, with regular medicine and with lvl. 1 medigun.

Level 6 illness:
-High dizziness, twice faster draining of fatigue, vomiting ,hardly walking, extremly high tempature, rare hallucination.
-Cannot be healed with low advanced pills, with low advanced medicine, with morphine, and wtih lvl. 2 medigun.

Level 7 illness: Possible death
-High dizziness, twice faster draining of fatigue (as in lvl. 6), vomiting, hardly walking, extremly high tempature, rare hallucination, very slow draining of health (1 HP per 3 game hours)
-Cannot be healed with advanced pills, with advanced medicine, with morphine and with lvl. 3 medigun.

(if you survive) Level 8 illness: EXTREME NIGHTMARE!!!
-Extremly high dizziness, quick draining of fatigue, coughing blood, extremly hardly walking, hyper extreme high tempature, huge hallucination (you can see demons, monsters, ghosts everywhere), draining of health (1HP per 30 game minutes)
-Cannot be healed by anything.

Theese are items for healing illness.

-To all healing items is possible to overdose and die or get bigger illness level instantly.

Pills:
-Comes in 3 versions: Normal, Low Advanced, Advanced.
Normal pills:
-Easiest to create of all drugs.
-Heals 5HP
Low Advanced pills:
-Medium difficult to create
-Heals 15HP
Advanced pills:
-Hardest to create of all pills
-Heals 25HP

Medicine:
-As pills comes in 3 versions: Regular, Low Advanced, Advanced.
Regular medicine:
-Medium-Easy difficulty of creation.
-Heals 10HP
Low Advanced medicine:
-Medium difficulty of creation.
-Heals 20HP
Advanced medicine:
-Hard to create
-Heals 30HP

Morphine:
-Has 2 versions: normal morphine and advanced one.
Normal morphine:
-Medium-Hard to create
-Heals 45HP
Advanced morphine:
-Hard to create
-Heals 60HP.

Medigun:
-Got infinite uses.
-Comes with 4 levels
-I created my own pic how it could look.

and even how it works:

Level 1:
-Hard to create
-Heals 50HP
-Can be used once per game day.
-Adds 12 game hours to illness change time.
Level 2:
-Advanced hard to create
-Heals 75HP
-Can be used once per 3 days.
-Adds 1 game day to illness change time.
Level 3:
-Extremly hard to create
-Heals 100HP
-Can be used once per week.
-Adds 2 game days to illness change time.
-Level 4:
-Almost impossible to create
-Heals same as lvl. 4, but only difference is that it can heal level 7 illness.

-To all of theese is rule if you overdose (if you don't follow the rule of the time limit (lvl, 1 - once per day) you get poisoned and your health will drain 10HP per gaming minute.

This is what is when your illness level change to bigger level.
Lvl. 1-2 = 2 game weeks.
Lvl. 2-3 = 1 game week and 3 game days.
Lvl. 3-4 = 1 game week
Lvl. 4-5 = 6 game days
Lvl. 5-6 = 5 game days
Lvl. 6-7 = 3 game days
Lvl. 7-8 = 1 game day

I will say all suicide techinques that could be and some will in stranded 3, some of theese are already in stranded 2.

Drowning:
I have nothing to say about this, because it is already in stranded 2.

Shooting yourself:
If you have gun, in inventory could be new secondary use, which would allow you to shoot yourself into head.

Cutting your wrist:
If you have any sharp melee weapon, you could have as gun a secondary use, which would allow you to cut your wrist.

Hanging yourself:
If you are near a tree and have rope, a secondary use button could pop into your inventory when you got rope choosed, which would make your character climb tree, hang rope to tree and hanging yourself.

Poison:
As drowning, this is already in stranded 2.

Medicine and medigun overdose: As I said in items spoiler, if you use alot of medicine you will get poisoned.

Jumping from high place:
I think I dont have anything to say about this one.