I agree that this sounds like a good addition to the game, however it does not fit the scale of the game. In reality flamethrowers and flamethrower tanks were used as parts of regular units. Fire bombers (or at least bomber equipped with incendiary bombs), though, were used as whole units and as such could be used in the game. The only problem is to find a place for them not already filled by another unit.
Keep in mind the scale of time within the game. Hours irl represent days in game. As such, burning effects would not be realistic.

Strat bombers r basically firebombers. Incendiary bombs r dropped on cities etc to burn the place down, ie Dresden or Tokyo. So they are already in the game really.

Flame troopers and tanks can attack the enemy relatively safe and with effect, in a case that enemy stayed in a stronghold.

Because flame deplete air which be complicated space for instance, pit, ditch, trench, shelter groove, etc. so enemy's soldier choke and die.

So I expect that this unit has the skill “ Ignores fortifications ”.

All in all, flame units wouldn't be an awful idea. Its just that they're too small a concept for this game. The smallest sized unit is a regiment, and after looking through the OOBs of all US and British units which saw combat in relevant theaters, not a single one had a dedicated flamethrower regiment. Flamethrowers were issued on a case by case basis at the company level, which is two steps down from a regiment. Similarly, flamethrower tanks were usually issued to tank platoon's HQ/reserve. At most a platoon had one, but it wasn't common.

Furthermore, to counter my own previous point, bomber carried bombs, and whatever kind they carried was mission dependant. There weren't any dedicated firebombers, and incendiary bombs were used not all that often (Dresden, Tokyo, Warsaw, Moscow).

It's also worth pointing out that the "bunker" structure in game isn't a single bunker. It's a series of bunkers spread throughout the area (think Normandy). As such, flame troops would be less effective than for singular bunkers.

The biggest problem that I see with this idea is that flame troops are basically different versions of other ones. For example flame troopers feel kind of similar to motorized infantry (good infantry damage poor armored damage) and flame tanks really aren’t necessary as medium/heavy tanks already deal decent infantry damage. As for flame bombers they sort of feel like a mix between tacs and strategic bombers. Not a bad idea but it would be hard to implement because they don’t feel unique and they’re too small of a concept as newbgamer said.

As newbgamer101 says, flame units are “ too small a concept for this game ”.
So I would admit it affirmative that such unit should not be added.
But do motorized infantry have means to choke and kill enemies soldiers within a fortifications?
Are flame tanks “ similar to motorized infantry ” from that point of view too?

Credit for the idea but I think flamethrowers are too micro to be relevant on the macro scale that is call of war. Ie can't we just assume infantry have dedicated flamers, if 1 infantry unit is supposed to equal 1000 men, and likewise it would be weird to have a division with 1000 flame troopers. This explanation might seem a bit lazy, but I don't really want the added hassle of deciding which of the many units get my precious research slot.

*Shameless plug* That's why we should have generals :D, ie a general might have a description of being 'invested in flame warfare' which would translate to an inital 2% buff to infantry damage and a 4% HP reduction and/or +4% fuel consumption (flame tanks go boom)