Well Borderlands has a crapton of parts when creating its guns. And those do more than any of Fo4's mods, out of Recon Sight and changing chambers.
The only tradeoff to the best mods avaleible is that they weight more. While that makes an Assault Rifle weigh 15 fucking Units, it's no biggie as i have like 700 CW on a Power Armor.
"Legendaries" could have even mimicked Borderlands and be a lot better. Not a stupid 5-20% of something, they could be a throwable pistol, an SMG that increases accuracy when aiming, 1% normal damage 500% critical, etcetera.
The more realistic option to this were the Unique weapons in previous games. While Fo3 was a bit uninspired, it at least tried. The originals had some like the Bozar and Turbo Plasma, and NV was even more awesome as almost every gun had an unique counterpart. That plus their mods plus the GRA versions made weapon variety huge. Not to mention melee/unarmed not feeling like an exploit of a single weapon (furious power fist).
Hell, I could be using the Vermin Rifle until I got a steady supply of stronger ammo, about Boulder City. I Fo4 there is 0 point in using anything else beyong that broken ass Overseer's Guardian and a fully upgraded Assault Rifle with the proper perks.