Its purely for flavour but Id like to see migration 'for the adventure' (for want of a better expression) included in the game, as well as migration 'for the new job/business opportunities.'
Id like to see in V2, if you form a new colony a broad sweep of POPs moving there, perhaps not in great numbers but enough to give that idea that the mother country is taking an interest in her dependencies.
A small amount of Aristocrats/Capis/upper class- to represent those who move because they are wealthy enough to do it/want to invest in new overseas schemes etc, the working classes as usual for obvious reasons and a few soldier POPs to simulate an officer cadre/regulars being sent to form/develop native troops.
I suppose workers from the colonies (again in small numbers) coming to the motherland would be cool-to simulate the importation of cheap labour (this could come with benefits/costs attached). This could also be modified by citizenship policy, ie its not so much of a problem if you operate a 'give me your tired, poor' etc immigration policy.
I enjoyed seeing a variety of POPs in places in V1, made the game seem more 'real'.