So there's good news and bad news. The good news is the skill system works fine, you can unlock skills which allow you to unlock skills linked to it, you know, typical skill-tree stuff. Next we have cooldowns and skill ranks implemented also. So you can unlock a weak skill and make it stronger later.
So what's the bad news? Well unfortunately it seems I can't figure out how to add attachments to weapons. So if I want a Weapon-tree system also, I'll most likely need to wait until the mod tools supports that.
Overall though, things seem to be progressing nicely, and I'll be working on a dialog system next, to make quests more interesting.