If “One of the world’s certainties” failed, this proposal does nothing.

Create an new rule called Demographics:

There are a number of Monster Species (or just Species for short) which are publicly tracked, each which has a distinct name that is flavour text. Each Monster Species also has a publicly tracked Classification that they belong to, which has a distinct name that is flavour text and cannot be the same as the name of a Monster Species. Any number of Species may belong to a given Classification.

Each Baron has up to one species that they belong to, which is publicly tracked. At any time, a Baron with no Species may set their species to be an existing Non-Royal Monster Species, or they may instead add a new Species to a (new or existing) non-Royal Classification and set their Species to that.

Add the following species and classifications:

Classification Royal: Demon
Classification Humanoid: Goblin, Minotaur
Classification Undead: Ghost, Vampire
Classification Formless: Slime, Mimic
Classification Outsider: Imp, Elemental
Classification Animalistic: Wyvern, Phoenix

Create a new rule called Migrants:

There exists a kind of Resource called a Migrant. Each individual Migrant must have a subtype, and that subtype must be one of the publicly tracked Monster Species as defined in the rule Demographics.
Any Migrant in a Stockpile is referred to as an Unsettled Migrant, and if a Baron would ever have more than 8 Unsettled Migrants, they must choose which Unsettled Migrants to remove from the game until they have 8 or less.  No Royal Migrant may be an Unsettled Migrant, and such a Migrant is removed from the game if they ever become Unsettled.

There exists the Accept Migrants Labour Action, which has the effect of adding two Migrants of any Species to the Stockpile of the Baron performing the action. There is no obligation for the Migrants gained by this action to be the same species as each other. This action is not subject to being Taxed.