The BIG HUGE feature request that you ought to be considering when it comes to Conventional Weapons is . . .
Weapons which have a Sub Weapon Attached to them.
M-4 Carbine with M203 Grenade Launcher
M-41 Pulse rifle with 30mm Grenade Launcher
As simple as it sounds though, I can understand why its not implemented yet - because I can see how it isnt really that simple to implement. (but it would be possible, just you couldnt separate the weapons from each other, even back at the base - its effectively one weapon which has another weapon Linked to it, and you can switch modes which switches it to the other weapon).
My suggestion if it was ever attempted, would be to just have an auxiliaryWeapon: option for a weapon which refers to a different weapon which is not a standalone and has no weight or size/storage stats.
The Auxiliary weapon would be accessed from the Firing Menu (aimed/snap/auto/etc) . . . something like "Swap Weapon" (which could even have a TU cost associated with it, representing having to flip a switch or reach for the other weapon's trigger).
Then the Weapon is effectively treated as if it is now the Auxiliary Weapon >>> the firing menu has firing mode options for the Aux Weapon, the Inventory still treats it as if it was the full size weapon but regarding ammunition it shows and deals with the ammo in the Aux Weapon rather than the primary.
(going one step further, though unnecessary, it could also change the sprite of the weapon showing the Auxiliary Portion of the weapon highlighted in Red or something, to indicate that part of the weapon is now selected >> I guess that could be the Sprite for the Aux Weapon itself though modders would be responsible for making those sprites).
(EDIT - - - Of course the easy way to do that is just have the Auxiliary Weapon itself have the same stats as the Primary Weapon . . . having the Primary one link to the Auxiliary, and the Auxiliary one have a revertsTo: option so that the weapon "transforms back" into the primary in any situation where the weapon has to go back into inventory shelf mode. That way switching an M4 Carbine to its M203 launcher isnt going to put it in the Base Inventory as an M203 Launcher . . . it just goes back on the shelf as its Parent weapon. The reverting itself could be done automatically . . . but being able to choose which thing it reverts to can give modders evil ideas about weapons mutating and transforming into other things after every mission)
The primary reason this would be beneficial for modders is you can build Underbarrel attachments based on advanced technology . . .
Using Xcom tech as an example . . . Mounting a Plasma Pistol to the bottom of a Rifle.
Or mounting Special Purpose auxiliary weapons to the gun.
Taking this a step further, down into the depths of hair spaghetti monster hell . . .
Modular Weapon Upgrades
Which work like the above but rather than linking the weapon to another weapon
It replaces or changes things about the existing one, transforming it into something else.
Example - Laser power booster for a Laser Pistol, making it look like it has a Silencer on the end of it basically. Increases the size of the laser pistol to 1x3 and increases its damage. But if you take it off it turns it back into a regular laser pistol again.
(in a similar vein you could make it to where the Laser Rifle itself is just a Laser Pistol with an attached Power Booster and Fitted Stock - - - like in the 90's incarnation of the movie Lost In Space where they had "build a better blaster" kits)
Im not saying these are feature requests, just one possible direction to take things (eventually, sorta, maybe).
Like the idea I had for Overpenetration.
