Are you going to add the ability to make custom advancements in 1.12?

You can already do this in vanilla... Just look up a tutorial for custom advancements.

I would recommend using the mod Triumph. It allows you to fully manipulate advancements and is quite robust.

Here is a link:
https://minecraft.curseforge.com/projects/triumph

You can already do this in vanilla... Just look up a tutorial for custom advancements.

It's per-world, though. Unless I'm missing something it's not possible to do it in a modpack/resourcepack.

I would recommend using the mod Triumph

Which is closed-source. An open-source alternative doesn't exist, apparently.

Not going to be a thing in 1.12, could maybe be a thing in 1.14 but no promises. I also recommend you use triumph.