US: remove the T20 and m2 carbine from the tech tree, but replace it with a rifle BAR with a bipod (of course keeping the br3 bars in the tech tree as lmg’s too). As it wasn’t just designed to be strictly a light machine gun, but to also be an automatic rifle to clear trenches.
Germany: controversial but putting the fg42’s on the lmg section but replacing them with the mkb and mp43 as SF rifles since the mp43/stg had squads with the purpose of every soldier being equipped with them.
Russia: get rid of the avs36 and avt40 but putting rather the a pps or ppSh in their place since in the later phase of the war, there were squad structures with each soldier except for the machine gunner wielding a pps or ppSh. Especially in the smg companies and sapper squads.
Japan: removing the type hei auto and replacing it with another type 100 late and adding another rifleman squad for Japan (make this squad an imperial guard unit) since the units who mostly got these guns were the imperial units and the teishin Shudan. And Japan needs a 3rd rifle unit in the tech tree anyway.
All removed weapons from the tech tree could be kept as legacy guns.