For the burner, I was thinking it'd be a great secondary. Say your sentry gun gets destroyed in combat. Assuming you're standing next to the gibs and no one has collected said gibs, you can weld the building back together.

The catch?

A destroyed and re-welded building can be rebuilt for a fee of 50 metal to compensate for destroyed electronics. It rises with only 2/3 of it's total HP and cuts the building's effectiveness (1/2 Sentry ROF, Dispensed Ammo Quantity, and Teleportation rate) and cannot be upgraded or repaired to full capacity again. (Level 3 Sentry Gun has 216 HP, welded it has 143.) It can only be welded back together once, and the second destruction of the building will make the gibs worth half as much as their original value. You cannot pick up Welded buildings in a toolbox. Welding the Sentry/Dispenser/Teleporter back together would take a timescale of 5/4/3 seconds, respectively. You cannot weld a Teleporter back together from it's entrance if the exit is destroyed, and vice versa. You can weld other Engineer's buildings back together, but the owner can destroy the building (with the Detonation PDA) after you weld it. Welded Sentry Guns can also be wrangled and repaired by the Rescue Ranger, but cannot be Teleported away (due to the toolbox.) In the PDA build menu, if a building is welded, it would indicate such by saying Welded instead of Built.

Operation:
M1 to Fire (Welds destroyed Buildings if available, else lights the burner)
M2 to Extinguish Allies (Deals no knockback to enemies, like the Manmelter's M2)

As for weapon attributes? I don't know what to put for damage output, but I imagine that you could light Spies and other enemies at close range, but not even close to the range of a Flamethrower. The weapon's ammo capacity is 100. Welding together a building uses 20 units of ammo per building (with 50 metal.) Extinguishing an ally uses 30 units of ammo. Ammo consumption upon regular operation (M1 with no building to weld) would likely drain ammo more slowly than the Flamethrower in exchange for a smaller flame.