Here are some policy ideas which I think would be cool to see in a Mod or in Democracy 3:
Progressive Income Tax - as a default, the income tax which is built into the game cannot be cancelled, and raising it to very high levels will make socialists happy, but tends to irk the Middle Earners since they are hit by massive income tax as well as the very Wealthy. In my opinion, the reason why is because the game simulates a flat income tax, say, for example 20%. However, most income taxes do not function in this way and are progressive, graduated systems designed to levy a higher percentage of taxes on high earners than on Middle Earners. A great way to allow prospective socialist governments to avoid hatred from the middle class with their policies would be to allow the player to set a separate rate of income taxes for each of the three primary wage brackets/classes.
Immigration Quotas - another dimension of immigration controls. Suppose as a country you wish to have moderate border controls but still manage to prevent a massive influx of immigrants to your country. The natural answer would be immigration quotas, a less extreme way of limiting immigration. Of course, quotas would be less effective as your level of border control decreases, and less necessary as it increases, so it is somewhat of a Catch-22.