The I-Go has a prototype that could be added, that would probably be a bit better considering it’s using a higher-velocity gun taken from the Vickers Mk. C that they imported as a reference for the I-Go.
The Ha-Go has four distinct prototype phases. The initial prototype, the second prototype modified with a lighter weight, the third prototypes modified with the distinct side “bulge” and commander’s cupola, and the last prototype with the rear machine gun turret installed which made it indistinguishable from production versions. There’s also the Ke-Ri, which was an attempt to place the Type 97 5.7 cm tank gun inside of a normal Ha-Go turret before they decided to just outright replace the turret with one taken from the Chi-Ha in the later Ke-Nu.
The Ke-Ni B could be added and the current Ke-Ni could be renamed the Ke-Ni A. The Ke-Ni with the prototype 47 mm gun could also be added.
The Chi-Nu prototype could be added, though it would be effectively the same aside from aesthetic differences and using the Type 90 7.5 cm instead of the Type 3 7.5 cm. The Chi-Nu with the Chi-To turret could be another addition, though imo the current Chi-Nu II should be replaced with the Chi-Nu/Chi-To combination and the Chi-Nu II made a regular tank between the Chi-Nu and the Chi-To in the tree.
The Ho-I could have its prototype added, which would be more poorly armed and armored. The initial prototype used the Chi-Ha instead of a modified Chi-He hull, meaning the front was only 25 mm thick, and it used the Type 41 7.5 cm gun instead of the Type 99 7.5 cm gun. There was also a later modification of the production version that installed the prototype 5.7 cm (ko) gun, which would serve as the middle ground between the Type 1 47 mm and the Type 3 7.5 cm in terms of penetration.
That same 5.7 cm gun was also installed in the first prototype version of the Chi-To.
The Ho-Ni I prototype could be added, though like the Chi-Nu it would largely just be aesthetic. The gun shield was a little less protective than the production version, though not by much.
The Chi-Ha was used as the basis for the TG and GS vehicles, and at least one was experimentally modified with a dozer.
There’s the second prototype of the Type 87 RCV that could still be added. We currently have the initial prototype.
The Type 89 FV was experimentally fitted with the Type 92 mineroller attachments, though this never reached any production vehicles.
The earlier prototypes of the Type 16 MCV and the Type 10 TK could all be added, both are missing a few.
The Type 61 TK could have its STA-2B and STA-4 prototypes added. The former would be a slightly buffed STA-2 while the latter would be practically identical to the Type 61.
The Type 74 could have its STB-4 prototype added.
The Type 90 has five prototypes that could be added, of which the first two used the 105 mm JSW gun.
The Type 60 SPRG has six prototypes that could be added. They would be more lightly armored than production vehicles, and the first four prototypes (SS-1, SS-1 kai, SS-2, SS-3) all used the M27 rifle, which means they would have 127 mm of penetration even with their best available HEAT. The SS-4 and SS-4 kai both used the M40 that formalized as the 106 mm rifle on production vehicles. The SS-4 prototype was later modified as an ATM carrier with two Type 64 ATMs, which could be a neat meme.
The Type 75 HSP prototype could be added and would be a general meme, since it would have an open turret.
The Type 87 AW could have its AWX prototypes added, of which there were two prototypes in three versions: the first fitted on a Type 61 chassis, the second fitted on a Type 74 chassis, and the second later modified with plates for the Type 92 mineroller.
An M24 Chaffee was later modified at one point to carry four Type 64 ATMs on the sides of its turret.
Those are all of the prototypes of IJA and JGSDF production vehicles that I know of.
