So after taking half a year off my texture pack, I've decided to take a look around and see what I missed.
I've noticed an "advancements" folder in the .jar folder and I looked through the code. I was wondering if it's possible to do custom icons or even custom advancements within a resource pack before I waste time tinkering around with it.
Yes and no.
No it's not possible to do custom advancements with a resource pack. Those files aren't actually read from a resource pack by Minecraft even if they're present.
On the other hand, yes you can do custom advancements from a Data Pack. This is something new. Remember how it used to be possible to name a resource pack "resources.zip" and put it int a world save so it loads automatically? Same idea here. Data Packs are linked to the world and put into a particular save. They're basically structured the same way as resource packs, though they can only be folders (not .zip files) and must be placed in /world_name_here/datapacks/ in order to work.
I hope that helps you.