For pack dev's to have a command executed when an advancement is triggered.

I'd really like it as a trigger for Darkhax's Game Stages mod, but I can imagine other cool ways to use it too: give players information, rewards, lore, spawn mobs for battles, teleport, etc, etc.

Yes please!

Sorry, I forgot about this. This can be done with vanilla advancements and also with the Triumph mod which basically just gives easy access to vanilla advancements to modpack creators.

Covered by other mods.