M79 type grenade launcher

[N1ghtmar3]

I'm not sure if this offer will appear in the release version of Underrail yet, but maybe you'll consider it in Infusion or in another paid DLC for the original game (I'd gladly pay even $20 for expanded arsenal and new crafting options).

M79 type grenade launchers came to my mind as a welcome item during the third playthrough as a stealth sniper. 
The main feature of grenade launchers (as combat utility) is the ability to launch a projectile at longer range and with greater accuracy than a simple grenade throw.
It is also possible to inflict mechanical or critical damage on a direct hit - can help against large heavily armored enemies.

This weapon will also greatly expand the useage of gas grenades, which, when fired from a grenade launcher instantly creates a gas cloud and can cause certain bad effects.

As ammunition, you can use grenades converted through a recipe into grenade launcher charges. Also, can be obtained from security robots (instead of stun grenades), grenadiers and etc.

For balance, using a grenade launcher sends the remaining grenades on cooldown.
It is also possible to craft this weapon and add modifications. For example:

Longer barrel - +30% to firing range, +10 AP cost
Cartridge case for the buttstock - -1t cooldown reduction

[Barry]

Best chance is in Infusion; setting up the sprite sheets is a tedious process because the same model is screenshotted over and over but with different poses and 8 different directions. For example a character can have a sprite sheet for moving, attacking with a rifle, attacking with a pistol, attacking with a sniper rifle, attacking with a crossbow, attacking unarmed, moving while having a pistol equipped, moving while having a sniper rifle equipped, moving while having a crossbow equipped etc. Setting up the sprite sheet for a grenade launcher would require a bunch of different sprite states (being idle with the grenade launcher, attacking with the grenade launcher etc) that will have to be attached to every humanoid character model in the game that can use guns to avoid janky animations and invisible sprites.
Infusion will have 3D models though, so chances are it will be as easy as setting up the pose and animations that will be used by most humanoid characters.