Well, unfortunately I'm not sure how far we can actually get in terms of creating new items/weapons/abilities/traits, because a lot of the stuff is located outside of the main/mod directories which means that it would be outside of what a mod could do at the moment.

That being said, I've found some ways to implement/change some skills and weapons to a certain degree.

In your case, start by going to wherever your Extracted Divinity folder is and search in the following folder for:

Extracted Divinity\Public\Main\Stats\Generated\Data\Weapon.txt

In this file, you can find a weapon that is similar to the one you want to make and then copy and paste it. Then change things about it. For example,

new entry "WPN_CraftedAxe_01"
type "Weapon"
using "_Axes"
data "Act part" "1"
data "Requirements" "Strength 6"
data "Damage" "2"
data "Damage Range" "3"
data "DamageBoost" "-10"
data "CriticalDamage" "1"
data "Durability" "7"
data "Value" "5.5"
data "ComboCategory" "Axe"

Take this and you could change it to...

new entry "WPN_DexAxe_01"
type "Weapon"
using "_Axes"
data "Act part" "1"
data "Requirements" "Dexterity 6"
data "Damage" "2"
data "Damage Range" "3"
data "DamageBoost" "-10"
data "CriticalDamage" "1"
data "Durability" "7"
data "Value" "5.5"
data "ComboCategory" "Axe"

Save the file and put it in your C:\Program Files (x86)\Steam\SteamApps\common\Divinity - Original Sin\Data\Public\NAMEOFMOD\Stats\Generated\Data
(You might have to create the folders)

But wait, there's more!

In order for your weapon to even appear in the game and to test it, you'll need to do two things!

First, you'll need to link it to... I'm not actually sure what it is exactly. Go to your
Extracted Divinity\Public\Main\Stats\Generated\Links folder and look for Weapon.txt

Open it and search for the weapon you had previously copied. In this case, we used WPN_CraftedAxe_01.

Copy and paste the entry and change it to the name of the weapon you changed it to. In this case, WPN_DexAxe_01.

object itemstat "WPN_DexAxe_01","9728e267-a187-4a46-9bb4-5db3478e0fbf","",0,0,"",-1,-1,-1,0,-1,-1

Save and then put that file in: C:\Program Files (x86)\Steam\SteamApps\common\Divinity - Original Sin\Data\Public\NAMEOFMOD\Stats\Generated\Links

Once that's done, we need to test it! Easy way is to make the weapon as the starting equipment for your characters. Find the folder: Extracted Divinity\Public\Main\Stats\Generated\ and look for Equipment.txt

In it, let's give the ranger the dex-based axe we created. Search for Class_Ranger

Add a new line or change one to say:
add equipmentgroup
add equipment entry "WPN_DexAxe_01"

As before, save it in your C:\Program Files (x86)\Steam\SteamApps\common\Divinity - Original Sin\Data\Public\NAMEOFMOD\Stats\Generated

Load up your mod and try it out.

Disclaimer: Like I said, it only works to a certain extent. If other people know better ways... please share! In fact, the axe doesn't even have a name. Probably because the name references the English.xml or something (which is outside of the bounds of mods as far as I know).

You can change the damage, damage range, etc in that Weapon.txt file.