Covetous Shen -though I sense his backstory may be intended for an expansion set
Lynden, the Scoundrel
Maltheal - or however one spells the name of the Angel of Wisdom who is strangely absent
Natalya - she always was the most mysterious character in D2
And, in the "ancient history category": BulKathos
Also, the family of Wirt and Squirt. Surely, they must have some odd cousins or uncles. um... Dirt? or maybe, Pert? or the mentally-challenged Hurt? or, the bottomless dancer, Shirt? or, the bottomless pit, Burp?