Marxism DLC.
I'm serious (besides the enormous memery it would generate). A DLC devoted to discontent, socialism, workers rights, rebellious intellectuals, terrorism, anarchism, various branches of left side of the spectrum welfare state, and obviously 17 - style revolution and its fallout.
Nationalism DLC!
An expansion devoted to national culture, fervour, identity, freedom, intolerance, ethnic relations, Great Men, national literature and art, and lategame ideology of Really Intense nationalism.
Colonialism DLC
An expansion devoted to various lighter and darker aspects of managing oversea colonies. Ideally (and tactfully) it would cover both sides - many events and mechanics involve relations with the colonized cultures, and their point of view.
Now that we have covered main sources of 20th century misery (I assume Capitalism is going to be the focus of base game), let us have obligatory Flavour Packs!
Minor Flavour Packs:
- Europe (the big question is would it be split up for individual powers)
- Islam (main focus on Ottomans, Persia and Egypt)
- Russia
- USA
- Latin America
- China
- Japan