Would also be nice to support any tag key (instead of just blocks, fluids, items) now that they have the registry key too.

I wonder how useful it is to reference registry entries directly. After all, if you have a direct reference to them in your datagen, it's quite likely that the item is from your mod or vanilla, hence it will be available at runtime. I am saying that an item_registered condition might not be super useful. Thoughts?

However, general tag key conditions could be useful I think.