I was thinking MA would literally play totally different from Zerk instead of just being a faster version. Like that jump attack thing, a jump attack could instead produce a jump kick that knocks zeds over or sends them staggering backwards. The class wouldn't be about solo'ing big zeds, it would literally be constant sprinting around, a few slices here and there, kick a zed away from a teammate, race over to another teammate and slice up the crawlers, etc. Like you said, light attacks are fast and do good headshot damage, and the attacking from behind would be cool. Heavy attacks would be really cool if they did a slight wind-up animation AKA Martial Artist. So instead of a Zerker who pumps out attacks rapidly and savagely, the MA would be a light, fast and squishy target who needs to keep moving to survive. Delivering mostly light attack combos and occasional heavy "special moves" like a sweeping strike that has a huge arc, or a jumping stab that can stun on headshot, or a leg chop that can knockdown zeds. And don't forget the jumpkicks. Oh yeah and fists instead of a knife so you can palm strike and spin-kick zeds in the face.
