I wonder if there's a way to turn off the padded populations feature after world gen? I'd be interested in playing a world where there were only the historical entities and the unnamed population counts only ever rose after a childbirth event. It would be a pretty sparsely populated world at first, but then the various battles between entities (where one entity is slain or retreats with or without a wound), would make more sense. Fortress Mode would quickly populate a world, as every Spring all the civs could add births to their entity count
There would have to be an average births per female per year aspect to each race, with a secondary percentage of stillbirths from unmarried vs married females added into the algorithm. For instance, human females -before modern medicine- had 1 birth possible every year, but an 80% chance of stillbirth due to various diseases and malnutrition. That dropped down to a 20% chance of stillbirth without malnutrition, which made unmarried females have a chance of 1:5 to birth a viable baby and married females have a 4:5 chance, so long as there was enough food.
Each year, the total available food would have to be compared to the population of each settlement in a civ to see if any would die from starvation - the subpopulations of "Child", "Adult" and "Elderly" would each have to be compared (pre-procreative, procreative, post-procreative) - Historically, Humans generally placed the needs of nutrition in this order: Adults, Children, Elderly. Only those in the procreative sub-populations would be able to be considered for generating new populations, with the Elderly often kept around for their use in childcare and their wisdom about how to best deal with the multiple challenges that may arise from year to year.
Anyway, it would be interesting to have a fortress populated by only historic entities, such that a poorly managed fortress with many deaths or well managed one with many survivors would significantly affect the population of the World as a whole. Likewise, battles of attrition would actually affect the World population, such that an Age of Twilight was once more something attainable.