Machine Gun Squads have the unique ability to build Heavy Machine guns. I think it is is an interesting mechanic that different Squad types can have a unique Engineer building that gives them extra functionality. But were you aware that technically there is another unique Engineer building, Ampulomets. Ampulomets were a Soviet incendiary lobber used in the early war period and were originally introduced back in the Stalingrad campaign
Ampulomets are in kind of a weird place in that certain Engineer squads both German and Soviet can unlock them as a building. I think given that MG squads can build HMGs, then Ampulomets should be made into unique buildings for Flamethrower squads. It’s literally a perfect and thematic match. Ampulomets would have to be removed from all miscellaneous Engineer squads and from all the various Event and Premium squads because I have noticed they can build Amps as well.
Next, the Ampulomet should be removed from Germany. I believe the reason why the Germans got them is because they didn’t really have a comparable weapon, and one can say maybe they were captured or Finland, Germany’s ally, captured and used them so hey Axis Alliance. But I think it is better to give each faction something unique
German Flamethrower Squads would have a new building, the Abwherflammenwerfer 42. It was a fougasse or flame mine and was based on Soviet flame mines encountered on the Eastern Front. I find there is nothing wrong with a little asymmetrical balance. The Soviets can lob flames, and the Germans are more defensive with a flame mine.
For the Allies they can have the Northover Projector. It was an ad-hoc incendiary grenade launcher made for the British Home Guard, similar to the Ampulomet
Finally for Japan this is quite possibly the most craziest idea……Fu-Go Balloon Bombs. These balloons were filled with incendiary explosives and sent from Japan to America on air currents in hopes they would cause damage. When built the Fu-Go balloon would launch straight into the air and out of sight. When an enemy walks into its target zone the balloon comes crashing back down. Yes this is a crazy idea but Japan didn’t really have a lot of incendiary gadgets besides grenades and molotovs 
