Wacky Weapons Workshop
Once you download and enable this mod you'll be able to attach most of the game's gun mods to most of guns in the game. This opens up tons of different combinations and it ensures that you'll have a diverse set of weaponry in your arsenal.
There are few other perks too. The mod changes the construction cost of all available gun mods to remove all perk and material costs. In other words you no longer have to burn your precious materials on your gun mods.
The mod also changes the names of gun mods to include a description of the gun type they were originally for. A nice touch.
There are two versions of this mod. There's one for the base game and there's one for those that have Fallout 4's Season Pass.