I've been complaining about things, and well, given what I saw somewhere else about Brad's original idea for GalCiv3, I think I can suggest an idea finally. I'm saying 2nd XP because the 1st is in development, and resources are limited. Idea for this comes a bit from Endless Legend
You would have five pops. Every time you gain 1 pop point (1 billion people in current parlance, you would place them in one of five population groups) Each time you gain a pop, you will have a popup to pick what type of pop (unless you want the governor to pick for you)
Farmers: increase overall population gain. Provide a inefficient amount of base research/wealth/manufacturing if more farmers than farms. Can be converted to other pops without penalty.
Soldiers: Spent on invasion. Soldiering has an overall quality. Converting other pops to soldiers causes both a morale and quality penalty. Drengin pops do not receive this penalty (via a racial trait)
Workers/Researchers/Servicers; handle manufacturing/research/wealth. Converting a pop to one of these pops causes a minor morale and efficiency penalty. Coercive trait would remove the morale penalty. Ideology/buildings can reduce the length of the efficiency penalty
colony ships and transports can move pop from one planet to another. Transports would be required to invade planets.
Buildings: Buildings would improve the effeciency of an individual pop. Capitals/major buildings would provide "free" base pop. (an elerium research building might provide +1 free researcher at full efficiency)- excess of a given pop without the infrastructure will start providing diminishing returns. Higher level buildings can service more of a given pop. (encourages building tall)
Morale buildings improve morale which improves efficiency and pop growth
Starbases provide global efficiency bonuses
Such a system could also provide immigrant/captured pops, which could be handled however Stardock thinks best. In addition, you could also crib from Alpha Centauri and have certain techs for certain races unlock "superior" pops (I could see this for Altarians especially)