There are many vanilla advancements like "kill all hostile mobs" or "traverse all nether biomes" — and also many mods that add new "hostile mobs" and "nether biomes", but they cannot support the vanilla advancements together. The only way is to overwrite the advancement file, which can only be done once and by one mod.

I suggest adding something like LootTableEvents but for AdvancementEntry, which contains the Advancement itself and its Identifier, which will also allow us to select the advancement we want. It seems that ServerAdvancementLoader and PlayerAdvancementTracker are pretty similar to LootManager(Mixin). The only thing we'd have to think about a bit is sync with the client, but I'm not sure