There are plenty of possibilities for authors to include some previous well-known characters to game, with their own storylines and quests.
So far NPCs which coudl be easilly included, considering time difference of couple hundred years between games:
Viconia - elves and dark elves could leave more than 1000 years, so no surpise we coudl meet our belowed drow here
Hexxat - immortal vampire, no boubts she could meet us
Edwin (or Edwina if you perfer) - as a very proverfull mage - maybe he or she coudl prolong own life expectancy for couple more centuries
Whom else do you have in mind?