I would vote for WW3 around 1980 but maybe a Calm Before the Storm type build up giving Nato and WP the chance to pre-build their armed forces to their own liking (i.e. Soviet Union could decide to add more carriers or surface ships vs land vs air).
Then have the spark that ignites the conflict randomly generated so neither side know exactly which month year the conflict will start. It would be nice for the game to shift to a weekly time scale once hostilities commence but I'm just brainstorming here.
Also - it would be a challenge to model the electronic portion war. ECM, Awacs, heat seeking missiles vs radar, and chemical warfare. Still - something resembling Tom Clancy's "Red Storm Rising" would have appeal - but how to do that in SC3 - that is the challenge.
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After that - I was thinking of SC Global War (the World War One game) would start to look interesting set in the American Civil War. Infantry, cavalry, artillery, balloons, etc.