Rather than a familiar which deals direct damage, don't forget about familiars which have other effects. Rather than a berserker familiar which dealt melee damage (which'd be mostly useless, since 99% of things that're within melee range of a decent berserker is either a sc/fp or dead), something which either buzzed around their head and distracted them (attack at 90% frequency) or slowed them down (shot goo and any zed within X metres moves at X% speed) or heck, was a mini-shield which jumped in front of you and halved the damage you took from any charging specimen (crawlers, gorefasts, scrakes, fps).
Be creative
Maybe the sharpshooter familiar could have an adrenaline syringe, which would be activated and have a 3 min recharge or whatever, but would allow you to trigger zed time when you wanted (perfect for those tricky headshots).
Demo has a claymore familiar. When a zed gets closer than X metres, it detonates and deals a chunk of damage to nearby zeds in a 360 arc. Cooldown.
Firebug has a napalm familiar? When a zed gets too close, it swoops over and dumps napalm on it's head, and it instantly goes into the panic animation, crispens and takes a fire DoT?
Note: These are brain splurges, not particularly balanced whatsoever.
Could familiars be taken by other classes? Thinking particularly of the zerker running around with a healing familiar, and a medic running around with an armour welding familiar.