radical idea and discussion area
I know there's the official twitter discussion and most likely several threads like this so I apologize for making a new one, but in tandem with the twitter post I thought we could summerize all our thoughts in here. So feel free to comment on my suggestion and post your own ideas on flame thrower alt-fire. As it stands UWE is considering launching your entire fuel tank as an explosive combustion grenade, triggered by shooting it or on impact as the alt-fire. Sounds like a desperate last resort kind of thing to me not an alt-fire feature.
My suggestion would be to have a secondary launcher on the flamethrower. This launcher would contain a 3D-printer which would print a breakable capsule directly in the chamber. that capsule gets filled with the flammable fluid used in the flamethrower from the main tank. It is also filled at the same time with specialized nanites that are programmed upon entering the fuel capsule with their own activation codes. Upon pressing alt-fire the flame thrower launches the capsule with compressed air or possibly a small fume launch a la spud gun, it shatters on contact either with a foe or environment. Pressing alt-fire again sends the activation codes and the nanites in the pool of fuel ignite the fuel causing a triggered combustion.
Since there will most likely be a visual effect for 3D printing in the game, eg the armory etc. printing your weapons. I would suggest some of this effect be visible on the weapon as well as it prints the capsule. So folk don't wonder why there is never any loading of these capsules done by hand.
here is a crude design, you'll have to excuse the flamethrower design :D