Revamp and add style to the visuals of many of the classes. For example, every shaman race gets a unique bear form. However, all races of necromancer get the same boring lich forms.
Perhaps keep the lower level lich forms of a skeletal variety - but there are so many size and race accurate skeleton models already in the game that could be more accurately applied here:
Gnome (small) skeleton for gnome necromancers.
Male and female specific iksar skeleton models for iksar necromancers.
Human & Erudites have your standard sized skeleton lich form.
Dark Elf would have a slightly smaller skeleton form.
As a necromancer levels up and unlocks higher level lich spells, the visuals could evolve:
Male erudite:
http://wiki.project1999.com/A_fading_erudite_apparition
Female erudite:
http://wiki.project1999.com/A_spirit_of_the_old_city
Dark Elf Male:
https://wiki.project1999.com/An_enraged_vampire (or spirit type similar to erudite)
Dark Elf Female:
https://wiki.project1999.com/A_soul_temptress (or spirit type similar to erudite)
Iksar Male:
https://wiki.project1999.com/Lord_Rak%60Ashiir
Iksar Female: I'm sure there is a female version somewhere in the game.
Gnome Male:
https://wiki.project1999.com/CWG_Model_XC
Gnome Female:
https://wiki.project1999.com/CWG_Model_XB
Human male / female: a spirit form similar to that of the erudites.
Also, there are SO many badass undead NPCs in the game that it makes no sense for necromancers to not acquire different pet types as they level up:
festering hag, mummy, zombie, ghoul, jack o lantern, iksar skeletons, undead frogloks, sarnak skeletons, etc...
All have such unique animations, sound effects, and best of all are already in the game (classic).