Deus Ex Machina in an exploration into a fan remake and modernisation of the original in Unity, redesigning systems and game mechanics to conform to modern iterations of the franchise and the genre as a whole. Not designed to be an exact remake of the original but instead and evolution of it with the intention being at this early stage to produce a reimagined Liberty Island as a portfolio piece.
In my search for inspiration with a new design for the Assault Rifle, I came across an interesting piece of concept art. It shows a highly modular weapon design with a grenade launcher as one of the modules.
From an immersive standpoint, it makes sense to have it as an attachment because when an organisation equips it's personnel with weapons, cost is just as much a consideration as the context for why this weapon is chosen and the roles the user will be fulfilling when using it. As such it wouldn't make sense to equip every Assault Rifle with a grenade launcher if the launcher is never going to be used by the user, as is with the original.
Similarly for roles requiring the grenade launcher, such as operations involving an attack on an objective, the apprehension of renegade agents in fortified locations, or in areas where there is a high likelihood of attack from an equally equip enemy force then they can be deployed. These seem like roles for special forces.
From a reality standpoint, grenade launchers increase groupings due to the added weight on the front of the weapon. Not having one equipped by default is a sure fire way for the organisation to improve personnel accuracy at a saving, and relegating its use to the special forces trained to account for this.
Another weapon that would benefit from having this as an attachment would be an Assault Shotgun, where the grenades are drawn from the same ammo pool as the ones used for the Assault Rifle. From a gameplay standpoint you would be allowing the player to make a choice about which weapon they want to equip it too when they first come across it which allows them to tailor their weapons to their play style and improve gunplay, with some rather hilarious and fun situations and firefights as a result. Finally it streamlines the user experience by not having to swap between to two weapons to engage soft and lightly armored robots mid-firefight.