Oh man, I love alternative history!
I have a heap of suggestions and stuff:
Civs: You seem to be neglecting a few large parts of the world. What if the League of Nations went downhill after the neoVisigoths, Hertfordshire'eans and the nations of Unigov left and ended up a confederation of bickering, paranoid, arms-racing nation-states. Perhaps they could be accompanied by various African colonies.
The hallmarks of the League of Nations could be varied troop and vehicle types, strong navies and efficient industries, but a lack of natural (luxury/strategic) resources.
What if (no 2) South-East Asia united under a single banner? The unification could have started as a means to resist the European colonial powers, but continued through power hungry leaders keeping the countries together in order to resist the Sino-Russo Alliance? An amphibian blitzkrieg (could have) lead to the annexation of Australia and New Zealand.
The hallmarks of the South-East Asian Entente could be big populations/pop growth, good guerilla/amphibian/jungle troops and resources but lack of heavy industries (atleast in the beginning).
Terrain: On a planet torn to shreds by war, there must be huge tracts of land rendered unusable by warfare, ie no-mansland. Some new graphics for burned forests and jungles, trench-ridden-mortar-pounded grasslands/plains/tundra/desert and dito hills/mountains. Productivity would be zero to near zero and defence bonuses reduced.
Units: What better units for an alternative history scenario than the ridiculosly expensive but oh-so-useful desperate units of long wars?
Floating fortress v1: immobile naval unit, capable of transporting and repairing multiple aircraft AND ships (though the repairing part might be unbalanced). Can only defend, but can bombard tiles/cities/units. Built using several fishing boats (or maybe special single purpose work-boats) and at a hefty cost in money. Perhaps a limit of 1 or a few more, buildable only in ocean to balance.
Floating fortress v2: Dead slow (1 move) naval unit, works as a medic for ships in adjacent tiles, can attack naval units, can bombard tiles/cities/units.
Again, perhaps a limit of 1 or a few more.
Hovercraft transports: Not alot to say. Transports units on land, coasts and maybe oceans.
Hovercraft tanks: Simple tank-ish units, can traverse and, coasts and maybe oceans.
Hovercraft artilley: Artillery unit, can traverse and, coasts and maybe oceans.
Just a few of my suggestions, might not fit in with your view on the conflict. Oh, and great mod idea!
*Looks a posting dates* Oh darn. Late post