Attachments exclusive to a particular weapon, or versions of existing accessories exclusive to a particular weapon, would enhance the appearance and feel of certain weapons and may provide useful enhancements.
Weapon-specific attachments already exist in-game; the L86A1 iron sights are visually distinct from the standard set, the F2000 scope is a weapon specific scope and is the only 1.6x optic available, and the quick/extended/drum magazine options when available are unique to every weapon. Other less-specific attachments include the PK-AS, OKP-7, and 1P78 exclusive to Russian/Soviet firearms, and the Osprey 9 for the Vector as an option for primary weapons.
Examples of weapon-specific attachments to implement are custom bipod models for the L86A1, MG36, and M249 to better match their IRL counterparts, the option for the classic Swarovski scope with “donut of death” reticle for the AUG A3, and the SUSAT scope for the L86A1.
Feel free to propose other weapon-specific attachments below.