Thinking about a weapon that uses metal as ammo to shoot molten spikes that deal fire damage only long enough to reward consecutive shots with minicrits on burning players. It gives rewards engineers that land their shots consistently at mid to long range, but does less overall damage than a meatshot from a shotgun, and is very punishing to miss shots. Engineers must also manage replenishing their ammo constantly, or switching to secondaries more often in emergencies.
Remember that Uncle Dane vid where he played pyro and made the wish that engineer could have the flaregun in his shotgun slot? Thats kind of where my mind is at with this, but the flare gun is made with comboing alongside pyros other weapons in mind, so it would need to work alongside engineers other weapons. The shotgun works well alongside the pistol and coil gun, because both the pistol and coilgun cover for the shotguns range issues, with consistent chip damage and mobility respectively. A theoretical "flare gun" type primary wouldn't have a range weakness, so a hefty drawback that leaves the secondaries as fallback plans is necessary.
