Having just written an essay on imperialism, it seems colonialism was inspired mostly by nationalist pressure and the need for colonies as a measure of status; and in fact the money spent on imperialism was greater than any income gained from it. While this wouldn't work very well in-game, perhaps colonies should have to be acquired in order to prevent unrest within the population at home, and to remove excess population after industrialisation, otherwise squalor and unrest would cause chaos. Also missions to acquire/settle colonies should come from pressure groups, for example in Germany the 'Kolonialverein'.