The roles I would like to see.
Sniper: low mobility, high damage, low attacks, high range
Machine gunner: High attacks
Light M.G.er: low mobility, low defence, high range
Sub M.G.er: low damage
Pistoleer: high mobility, high defence, low damage
Shotgunner: High damage, low attacks, low range, versatile(incendiary: highest damage; shot: knockback; slug: lower damage, higher criticals)
Melee: High mobility, High defence, low range.
Medic: healing, buffs
Engineer: static defences, debuffs(flood lights reduce accuracy, barbed wire/caltrops reduce movement, Tesla coils from Red Alert hamper longevity)
The forward firing turrets are a great idea, they are a really nice battle-field-shaping item. The engineer would be a great class for someone who wants to play master-mind funnel the enemy into every more elaborate snares and lures.
Demo would be best distributed amongst engineer and L.M.G., hand out a mortar instead of a light machine gun and you have area effects with fewer attacks, more damage if there is a different target in every affected space, but less if the enemy is spread out and less ability to remove specific targets. Engineers could get mines, and everyone could have grenades if they are willing to spend the cash...
I am kind of attached to my character, so I would rather not start completely from scratch, but I have no objection to a rewrite of the abilities to tone everything down and impose upper limits to possible bonuses...
A zombie ability you could consider is to have maximum damage from a single shot, it would make them sniper resistant and combined with a percentage reduction in damage or reduction in damage of each shot could limit the damage of high rate-of-fire types too. Just have something that everyone needs to beat down for a few rounds to kill, combine it with healer types that you would need to kill first and you have a compound enemy...
Armour is on my list, but first I want the perk to increase funds, and then I need the hat to prevent criticals, which are what will kill me, and then I probably want damage-per-hit reduction which doesn't need to be repaired, and there is that perk with the 20% chance to be missed, and the perk which doubles attacks, and I would love to get whirlwind back, but armour is definitely on the list...