^ Yeah I don't know I wouldn't mess with the damage or maintenance of weapons (it's not an armory simulator, soldiers are expected too keep their gear in top shape before a combat operation, and the non-standard parts do not strike me as something truly historically accurate - cosmetics yes, extra sidearm from "looting" - yes, but modifying your gun's workings? Not really, those are usually done by professional armorers, not soldiers in the field, I think). On the other hand, things that affect the character, that is those that simulate the process of "maturing" as a soldier should be in. Nothing major, just some perks that can give you a "slight" advantage vs an equally skilled player (twitch-skills-wise), if you know how to use them.
EDIT: Some "unlock" ideas might involve extra ammo pouches, extra bandage, bigger mag versions (where available, for the Thompson for example - but it must come with a disadvantage too, for example reload takes a little longer, and you run slightly slower because of extra weight), maybe worn out barrels for the Bren? (improves spread, but it also makes the weapon less accurate) and other indirect historical "boosters". Nothing that gives clear advantage in a gunfight, in terms of damage dealt.