some nice suggestion but lots of unrealistic ones too...
Zerker is like the brute who can take and inflict more physical damage than others classes. so he souldn't have less carying weight (he is like more muscular), however, he should loose speed (or stamania) whenever he carry a non melee weapon (out or not) exept light weight ones like handguns (2 slot or less)
also, adding stamania is a great idea, to all classes(only zerker would be stupid), but zerker would loose stamania faster when attaking because firing a gun take less energy than swigning a sword. while commandoes can run longer distances while carying assault (comandoes need a small buff too, maybe increased speed when aiming and crouching,)
Katana are usually weak and fragile swords that need a powerfull swings to cut tru hummans bones, maybe a little break gauge that decrease effecacity of katana over time, like when at zero zerker with a katana do same damage (but still swing faster) as other classes using a new katana.
chainsaw NEED a buff, but slow down a lot, but insta kill small trash with primary, like a shield. with secondary, should be faster, to melee lock scrakes on normal and hard, but not on sui and HoE
Each classes have easy zeds and hard zeds to kill, the only hard one to kill is the siren, i think zerkers should be unable to stun scrakes, or make scrakes AI better so they dodges zerkers hits with their chainsaw. that would be awsome
what this game really need is better AI, damn they are HUMAN CLONES, they need to stop acting like brain-less zombies. make some zeds smarter, make the stalker actually ambush you, make crawlers hidding in dark areas then silently jump on your back to bite your legs, make scrakes protecting himself with his chainsaw, make siren screaming to protect other zeds from explosives, even if there is no one near her...this would require more coordination and strategy to defeat em, like real clones.