Keep the current 4 major factions unchanged but separate the tech tree into several minor factions.
Each faction got its own weapons and squads.
But all equipment in the current major faction is interchangeable, and you can bring any minor factions’ squad into battle under the same major faction’s flag. So, the tech tree for minor factions do not need to be complete with all BR weapons that is not underpowered and fill all roles.
e.g. you can make an Allied BR3 loadout like this:
A British assaulter squad equipped with M1 Tompsons
A Franch guerrilla squad equipped with silenced Sten guns
A USA motorcycle squad driving Chinese Panzer 1
A British tanker squad driving their own Churchill III
This will give more space for adding some interesting weapons to tech tree, also can serve as a remembrance for those countries which fought in WWII such as China, France, Poland.
And (maybe not that good for many players) letting players take longer playing time to research the weaponries and equip an optimized army for high BR battle.