Let me make lists for each skill tree. I'm open to suggestions for more.

[b]Combat Skills[/b]

Whatever skill you use to fight or injured/kill someone or something 

[hider=Unarmed]
I have split unarmed/martial arts into two categories, kickboxing and grappling. Ground fighting is mainly influenced by grappling. Feel free to also mention specific martial arts just to flavor the character and keep things interesting. For instance if your character knows a little Muay Thai, do Muay Thai (boxing) 3.
[/hider]


[hider=Melee Weapons]
There are blunt weapons, bladed weapons, and stabbing weapons. Each are separate skills.
[/hider]


[hider=Ranged Weapons]
Ranged weapons can be anything from throwing rocks to firing a laser rifle. Some require strength or dexterity, but either way they require skill. There's throwing, slings, bows, handguns, rifles, machine guns, and energy weapons.
[/hider]

[b]Support Skills[/b]

Skills generally used outside of combat, though you could improve in combat. Not to harm.


[hider=Social]
Skills that give an edge when dealing with people. Charisma for convincing people amicably, Intimidation to scare people into doing what you want, and Deception for tricking people into doing what you want.
[/hider]


[hider=Wilderness]
Anything dealing with nature or surviving in it. Animal skills are knowledge of how to deal with animals, Survival includes building fires and shelters, Herbalism includes knowing which plants do what and what to avoid, and Hunting involves not only hunting animals but also cutting them up and harvesting what you need from their carcass.
[/hider]


[hider=Athleticism]
These are more like attributes, which I was avoiding, but without any of these I will assume your character is average in one of these. Strength to lift heavy objects, Endurance to deal with pain, discomfort, and fatigue, Agility to dodge or move quickly, and Dexterity to help land a shot.
[/hider]


[hider=Stealth]
There are three skills here. The first is, of course, Stealth which means sneaking or hiding to avoid being spotted, and Lockpicking which works for traditional locks, and Pickpocketing to steal right from someone's pocket without being caught.
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[hider=Technical]
Computers, for hacking and possibly creating programs, Repair, for fixing things, First Aid, for patching people up.
[/hider]

Magic is currently a mystery to your characters that will be revealed over time, so I will keep it a mystery to the players as well.