The archetype just tell the game to assign specific mesh parts to an entity in entity_class. Or did...
<archetype name="BanditLeader" male="true" hair_color="148,74,40" eye_color="68,68,60" skin_color="175,175,175" type="None" stance="Like" voice_set="trader">
<base_mesh_parts>
<part slot="head" layer="base" mesh="male_base_face">
<texture name="male_head" color="skin"/>
<texture name="male_head_old" color="skin"/>
<texture name="unisex_tattoo_tribal" color="15,20,25"/>
<texture name="male_head_beard" color="hair"/>
<texture name="male_shaggy_eyebrows" color="hair"/>
</part>
The problem is that the zombie archetypes are replaced with prefabricated meshes which look great but we end up with just the same looks. The models themselves will have to be manually edited. The hair and skin look like they may be able to have changes. It doesn't look to me like it can't be done with Z's but I'm not a Unity modder so I was throwing it up to you in hopes you might know.
And yes, the bandits and survivors from the creature packs are what I meant as well as from vanilla. Just for a little diversity.