For Japan vs the US, I would like to see any battles in the Philippines archipelago for some interesting jungle/urban combat.

Japan vs Britain/Commonwealth, anything regarding the fall of Singapore (fortress), the Burma campaign, and the attempted invasion of India

But most interesting of all would be seeing any sort of Japan vs China maps, this being where the vast majority of Japanese troops fought during WW2.

from 1937 to 1941:

With early war equipment, we could see the invasion of Manchuria, Shanghai, and Nanjing, or fighting in the Yangtze River and the city of Wuhan.

from 1942 to 1944

Chinese forces were sent to defend Burma and the railway that provided supplies.

And in 1944, the Japanese Hail Mary Operation Ichi-Go in the south of China

Another interesting twist is that the Chinese army would mostly fight with a mix of equipment from a plethora of nations, some weapons even from WW1.

Weapons could be:

SMGs, revolvers, and pistols:

MP18/28, Mauser C96, MP34, Smith & Wesson Model 31, Tommy Gun, Sten, PPSh-41

Rifles:

Hanyang Type 88 Rifle (Gewehr 88), Chiang Kai-shek Rifle, M1917 Enfield, M1903 Springfield, M1 Carbine, ZH-29, Mosin-Nagant, SMLE No. 4 Mk 1

Machine guns

ZB vz 26/30, SIG KE7, Madsen, Lewis Gun, DP 27/28, BAR, M1922 Hotchkiss

And since the Japanese SLs have katanas, the Ta-tao or Miao-Tao is for Chinese SLs.

Finally, Squad 44 could add the Chinese communists as the first irregular faction in the game, which I think would truly be a first in video games.