It could look the same like Helmet of a Pyro Spec for Heavy Helmet and something like Tanker, or Scout Spec helmet for Light variant. IC reason is kinda obvious, it's the same as choosing armor different from medium. Why PFC's would need Heavy Armor/Light? They want more protection/freedom of movement, also to the head (and hands, but that later
). I guess that every real modern army does something like that, if somebody receives heavy vest, they aren't wearing standard helmet.
Heavy Helmet should be limited only to Heavy Armor (with IC reason that they are supposed to work together, connecting with eachother for better protection) and wearing them should work like SG have to wear SG Harness to wear their optics.
Medium and light could be a matter of preference.
Medium helmet would be as it is, maybe could receive additional storage slot. Heavy Helmet would obviously grant more protection, but it would lose all storage slots, you couldn't perform CPR with it on and you would be of course bound to wearing heavy armor, maybe additional weak air filters for reduced damage from gas, but view reduced by one tile.
Light helmet could boost Accuracy (Something like small computer and hud helping marine aim), but would be weaker than medium helmet.