Association inputs have a very cool option to "Allow users to add new options.".However, these cannot be nested.
Example: I'm editing a Product, and have to choose a Brand. It's not on a list, but I can click the "+" to add it.
I'm now creating a Brand, and have to choose a Manufacturer. It's not on a list, but I cannot add it directly. Instead, I have to go all the way back to add it beforehand :(