The Type 56 105mm Experimental SPG was a slab-sided self-propelled gun for the Japanese Ground Self Defense Forces based on an M24 Chaffee tracked undercarriage that harkens back to the late WW2 Type 5 Na-To. As such it could be used as the next tier up in a game that goes beyond the weapons of WW2 and hews as tightly as possible to Japanese indigenous designs. Being an obscure prototype it would be an interesting addition to any game.
“JGSDF Type56 10cm Experimental SPG. Built on the constructional base of an American Chaffee, the Japanese built this in 1957. It used a casemate hull equipped with an M2A1 10.5cm Howitzer. The project was canceled shortly after testing [however] due to the use of the M44 and M52 [instead].” (Credit: Mai_Waffentrager, War Thunder Forums)