This is probably an idea that has been thrown out there before- certainly a recent post or two have essentially asked for demographics, but I dont think they really opened up to all the possibilities demographics entail.
So what are demographics?
Essentially, it is/would be the percentages of what races make up the population of a planet - 90& Terran, 5% Arcean, and so on (and could use just a simple 'race adjective' when creating a custom civilization.)
Okay, so how could they impact the game?
In a myriad of ways!
Imagine that you chose a policy that integrates the existing population (or those that survive the invasion) of a conquered planet as opposed to genocide - you now have distinct populations of different species with different interests and desires. Perhaps that Drengin scum you have allowed to live wants to reunite with their empire.
That situation alone lends itself to a lot of interesting ways of dealing with it- perhaps you do end up killing them, or committing them to slave camps, or 'reeducating them', or allowing them to live as is, to see the virtues of your culture, etc.
Or maybe all that tourism and influence your generating on your border world is causing an influx of migrants from the nearest few planets as visitors decide to stay due your lax immigration laws (or in spite of harsh immigration laws) and your now stuck with similar issues of how to govern them, or how to eject them. And if you let them stay, how will this effect cultural influences in your colony?
And then in a very different route, an entire branch of it could deal with native species when your settling new planets- more then a few events deal with native species, and adopting these populations into your Empire (but still as distinct species. maybe with the generic name "Native species of [planet name]", with that generic name being re nameable by the planets owner.)
To condense that:
-It alters game play by giving another potential avenue for ideology points, as well as gaining planetary bonuses and maluses.
-Additionally, it can affect the greater culture game by make planets more or less susceptible to flipping, and potentially lead the way powerful civilization wide bonuses and maluses depending on if the policies vary from planet to planet, or if you alter settings on a grand scale.
-Can add a new inward looking focus to game play, and not just the constant 'ever outward' push - but could easilyl be a system that could support either "wide' or 'tall' empire building strategies. There is tons of potential for strategic synergy with a system like this.
-Really, really, really great for RP purposes.