World War 2 eh?
I've been working on researching a Napoleonic-Cold War "Total War 2: Modern Warfare" game. 2 Campaign maps for Atlantic and Pacific Campaigns Campaigns so as to have some degree of detail (one map for everything is like world conquest for ants. It needs to be like 3x larger.) The Western Hemisphere one is pretty much focused on Europe in terms of factions although there are quite a few in the Americas. Africa is pretty much an open season for Imperialism outside of the Ottoman and Ethiopians. The Eastern campaign naturally gravitates around European interests in the Indian subcontinent, the Chinese sphere, and such.
Not that it's necessary to do that much.
I think there are like 65 units, but most of them would be used by many factions. Units fall into four tiers of advancement in my head. For France that would be Napoleon I, Napoleon III, Clemenceau, and De Gaulle. For America it would be Jackson, Scott, Pershing, and Marshall, etc. Basically Napoleonic, Post Napoleonic, Great War, Post Great War. Its quite probable there would be Early Units which can Armor upgrade the various guises of the 19th century. Late Units might do the various guises of the 20th century. In that scenario there's probably going to be another 15-30 units needed although if you want to focus on 20th century it wouldn't come up. Cavalry wouldn't need different stats, but infantry would. A Brown Bess and an Enfield are relatively similar but an Enfield and Lee-Enfield are very different animals.
Although obviously it isn't necessary to do all that, that's just what I'd been thinking about. World War 2 would need far less to be successful and you could change the factions quite a bit.
Very WIP French Roster Concepts
Skinning everything would be a mammoth undertaking, but it would be quite playable with the "wrong" skins.
Eh... too lazy to convert from .psd