You can't mod in ballistas that fire projectiles with ropes attached. There is no support or modding tools to make that. There is no support for an item that spans multiple tiles at all, really, and that's what a "rope attached" would require. 100% not supported in the game engine.
you can't make a machine gun or similar, Projectiles need to come from a quiver, you could name a quiver "magazine" if you want but you can't do anything to indicate the concept of a loaded weapon. You'd just have a dwarf with a "gun" bow and a "magazine" quiver full of "bullet" arrows. There's definite limits on modeling sufficiently bullet-like projectiles. If you make them bullet-weight they're not going to have the penetration power: there is no real override for projectile speed that can give a light projectile bullet momentum.
There's no way to attach a gatling gun or any other weapon to a carriage or platform. The best you can do is have a "gattling gun" animal which is self-propelling and self-firing, and you use breath attacks for the damage. The same with flamethrowers, there is no way to make a breath-attack weapon, but you can mod animals / races / or maybe castes to have a breath attack (like a dwarven "flamethrower" sub-race, but they'd have to be born as one or morphed into one)
Molotovs isn't doable either. There's no simple way to tell something to explode. Maybe possible with DFHack, but it'd be a nightmare trying to use scripting to determine whether a projectile had just been fired (it'd have to scan every single item on the map constantly every frame), what type it was and whether it should explode.
Chainswords would just be a regular sword called "chainsword" as there are no particular ways to mod it different to a normal sword really.
So, in other words most non-fantasy weapon mods are only going to amount to flavor text not actual ingame differences.