Options would be:
-
Stock: three variants minimum (CQC, medium and long range). Each option would affect recoil control, movement speed, ADS and others to various degrees.
-
Ammunition: AP, incendiary, explosive, high velocity.
-
Grip: recoil control, ADS, flinch resistance, among others.
-
Trigger Group: changes default fire mode. Examples: automatic -> single, automatic -> burst or single -> automatic. We could turn an M9 into a Beretta M93R-like or make an automatic Desert Eagle (GS Magna, anyone?)