I friendly disagree with the idea of having different races options inside the undeads and I do due two problems: the first and biggest problem is that this would take too much time and work to devs and in comparison with other races, undeads would like very superior compared with ofther races. How many ours of work would this take? three or four times the time taken for other races?more? I think this would be unfair and non-viable. The second problem in more abour the personality of the race because, undeads are not conceived as "everything that is dead and rised up again is an undead", but instead of it, they usually are conceived as a fallen civilization, servers of the darkness....and thats why I think they should be an unique model and based on humans. I really love the idea of that undeads are part of a fallen human kingdom and having that bond with their human past.
So, speaking about customization I only would like to have different body options of udead: maybe one more rotten with the bones and entrails in sight, one more like a vampire with the body less rotten...just different kind of putrefaction for your undead
Models of the various undead can also be used as corpses for in-game scenarios.