Have you decided on which factions you’ll use in your game?
Nope.... that will largely depend on what capabilities/features the SC3 scripting system has - something I have only dabbled with and really need to learn much better before trying to make this a legitimate campaign.
For the moment, the map is divided pretty much along provincial boarders. Historians consider the so-called Chinese warlord period to be pretty much over by the start of the War of Resistance with Japan. However, there were still very strong echoes of that period, both political and military. For example, many of the provincial governors were really appointments of former "warlords" by the Koumingtang (Goumindang) nationalist government. The KMT had to decide whether to defeat or subsume the various warlord armies and, if they felt they were trustworthy, it was always easier to make a warlord a provincial governor, complete with ready-made army.
So, by 1937, Nationalist China officially was pretty much all of China. But the reality was that true control exerted by the KMT waned roughly radially with distance from Nanking (their capital).
Superimposed on all this was the separate communist movement and the Chinese Soviet Republic which effectively controlled various territories around China.
It will be difficult to model this properly and historically in the game. However, I have the sense it can be roughly simulated through the scripting system and by treating provinces as minor countries. I have a feeling it could end up a pretty cool game, even if historical accuracy needs to be a bit distorted to achieve that end.
Anyhow, this is all far in the future.... assuming I can even accomplish it... I have SOOOO much to learn about scripting, etc...