To keep it simple, only modification of squads, no extra 20mm Rifles, or lighter mortars etc., as that would require remaking of lot of OOBs.
Standard rifle was Arisaka 6.5 mm, but Japan already began rearming into 7.7 mm version, because of too weak projectile in old model. Both were produced in vast quantities (3.5 millions), and rearming began:
The change of the rifle from Type 38 to Type 99 was carried out division by division, because the ammunitions must be unified in a division. At first, the divisions in Manchuria changed Type 38 to Type 99. Then, the divisions in Japan homeland changed. The divisions in the Pacific front were the last. However, the change of the rifle was not finished during the war. So, many infantrymen in the Pacific used Type 38 until the war ended.
So...
Japan begins game with standard squads, as they are now (Hard Attack 5, Soft Attack 20, and 17 weight), but those are squads armed with Tye 38 Rifle, and no longer "produced", new squad Type 99 is introduced, and all units in Manchuria, and Japan mainland are already rearmed to them. That would give interesting option, that, when Japan player will turn replacements for all units, including China, rearming of all squads should suck all armament reserve in a blink. I am wondering what happens, when units is supposed to get replacements, and squads are no longer in production? Anyway, new 7.7 mm bullet should increase firepower by about 10%, so 22 Soft Attack looks about right.
SNLF units had similar organization to infantry, so squads should weight the same, but units were small, and prestigious enough to get some rare weapon, like 13,2 mm HMGs, instead standard smaller calibres, or Thompson MGs.
However, there was an submachine gun, the only produced in any quantities in Japan, which is reported to be used during invasion of Southern China, by "Marines", and special, folding version, was delivered for paratroopers, and used in February 1942. Reported production of over 20k, should be enough to equip all SNLF, and frontline para troops, during war, so we can safely assume, this was a standard weapon of frontline "special infantry". Then, there was also spigot launcher, reported to be used by SNLF.
SNLF squad had much firepower. Mostly equipped in 8mm submachine guns, custom MGs, and spigot launchers. We can safely assume, that Soft Attack will be:
13 soldiers in squad, but in game terms squad probably include also 3 soldiers from "knee mortar" team, still mortar crew is not line infantry, there are 4 soldiers in MG team, and one is detached as runner for HQ, that leaves 7+commander to equip with submachine guns.
That would add around 8*2 (better fire-rate than M1) Soft Attack, so 36-38 seems right.
Paratroopers:
10-13 paratroopers per plane, suggest, that their squads were smaller. But since there was 4 people for Machine Gun (operator, assistant, and 2 ammo carriers), I am guessing, they just get rid of extra ammo guys.
Since they have very precise plans, and used flamethrowers in at least one operation, we can assume, that they get every possible specialised equipment to carry their mission. Including Engineer demolition Charges, and antitank "grenades". Plane jump means, they could not get much ammunition, but they surely should have better Hard attack. So HA 15, SA comparable to SNLF, maybe few points less because squad is smaller, also because need for conserving ammunition (28-34), weight 14-15.
Now, things begin to look interesting. On 12 May 1942 arrives to Japan example of German Schiessbecher. By August their copies are being produced, by October some examples reach Guadalcanal.
So...
Considering, that during August 1942-unspecified 1943 Japan produced around 49000 Type 2 Grenade Launcher, we can make assumption, that there is enough such devices, to equip ONE soldier in both SNLF, and Para squads. They could also shot AT grenades, and adding such hand-AT-grenades could be captured from Allies in Singapore, and Bataan in large quantities:
new type of SNLF squad should be available at September 1943, with HA 15 (standard for Allies), but there should be already enough Type 97 20mm AT Rifles, to add some to SNLF units. Standard organization was 2 such Rifles per Company, so HA in fact should be 25-35 and slightly better SA (Type 2 Grenade Launcher), maybe by 2 points.
Similarly for Para squad, HA is already 15, so we could increase it to 20 (Type 97 20mm AT Rifles were far too heavy for para drop), SA also +2.
And now... things looks even more interesting.
In 1942 also begin production of Type 100 Grenade Launcher, and in 1943, a new AT Type 3 grenade (first produced by Japan) begin production. Since there is both Type 2 Grenade Launcher, and Type 100 Grenade Launcher are already at full production, a new infantry squad should be introduced in January 1943, with standard HA 15, and slightly better SA (+2-4).
1944 sees introduction of Japan version of Bazooka Type 4 AT Rocket Launcher, and also new model of Type 100/44 Sub Machine Gun, with almost twice fire-ratio.
I would put first equip attempt after Peleliu, and before Iwo Jima, so in November-December 1944 new Para squad, with HA 55, and around 5 points more in SA should be introduced. In 1945, but still before Iwo Jima, so in January 1945, new SNLF squad, with similar statistics should be available.
And interesting future. Game so far do not force Allied player to haste, new Japan plane types are good, but when they will be available, more than 50% of plane production will be in ruin,
So...
as a penalty, if Allied player is "behind the schedule", in September 1945 all infantry squads could be equipped with Type 4 AT Rocket Launcher. HA 55, another extra grenade launcher could increase SA by 2 points.
More about it later...
What do you think?