Here's an idea.
A rocket module that instead of firing single large rockets, fires a volley of ~5 smaller rockets. But more than that, instead of just dividing the blast damage among a larger number of projectiles, the MLR does less blast DPS overall but the rockets have a chance to start fires. Think of it as instead of HE it was decided that an incendiary warhead was a better match for the smaller rockets.
So it provides the option to cause fires from a greater distance than flamethrowers, but is cumbersome and much more ammo hungry as well.
Not sure if the incendiary effect is currently possible to do with the modding system.
If I were to design it I would probably start with the rocket module, have it fire a volley (say, 200ms between shots) of 5 rockets doing 1/5 the blast damage, but then make the reload time longer for lower DPS and the ammo consumption greater in exchange for the possibility of starting fires.
EDIT: I don't see any special flags in the flamethrower data, so maybe blast damage is just inherently fire causing?
Yep, blast damage has a higher chance of causing fire.
Okay then, so simply breaking up the damage into smaller chunks inherently makes the weapon more flamethrower like :D
Another idea I had was instead of lowering the DPS as a trade-off, a possible version would keep the Rocket module's high blast DPS, but instead have greater ammo consumption.
Is the ammo consumption of a weapon an adjustable parameter? I don't see an entry for it so I guess it's a secondary calculated characteristic of a weapon?
Yes, indeed, the more individual hits, the greater the chance one of them causes a fire.
Ammo consumption is determined by the "clip" value, so how many shots you get for one ammo resource. And yes, it's definitely something that can be used for balance trade-offs design-wise. :)