I like the Red Wizard of Thay idea, but the Red Wizard is an outcast, for attempting to steal certain things from own mentor. He's under a disfiguring curse, requiring him to conceal his features under a robe and cowl, and is hoping to train young apprentices for several possible reasons;
1) he needs the money gained from tutoring, to pay for research into a cure / repay the offended master for the stolen items, in which case the master will cure him.
2) he's hoping to find an apprentice who can learn how to defeat the curse, and his 'lessons' will be geared towards that line of study. Even if the student can't do it by the end of the lessons, he will at least have helped advance the research into the cure.
A mix of both would be ideal, as the former Red Wizard wants to break the curse without repaying his debt, allowing him to pocket his tutoring fees.
Complications;
Does the former Red Wizard want to return to Thay, all sins forgiven? Or is he happier as a bigger fish outside of Thay, *not* surrounded by evil opportunistic wizards who tend to stab each other in the back for advancement and be way more powerful than himself?
Is he working for redemption, or are the Red Wizards (or, one at least) hunting him down, looking for their stolen property, or just out of spite? Did he sleep with the Zulkirs daughter, or, possibly worse, run out to escape an arranged marriage with the Zulkirs (ugly) daughter? Will the PC end up having to help him repel an attack by Darkenbeasts, sniffing him out for his old master? Will he, in the course of 'training,' instruct the PC in how to assume a certain form, which is, oh-so-conveniently, the form by which he is hunted, hoping to use the PC as 'bait' to draw out his own hunters?
How does this 'disfiguring curse' work? Is he Sheelba of the Eyeless Face, or Ningauble of the Seven Eyes? Is he all malformed, like a Picasso painting that's been left out in the rain? Does his appearance change randomly? Has he, for his dalliances (or lack thereof), been changed into a woman? Does he wake up each morning with a new face and body? Is he slowly changing into a Gibbering Mouther, and even in his, currently still humanoid, state, is he always accompanied by the sound of many voices muttering under his robes? The important part is that he wants the curse gone, and the curse prevents him from living in some sort of settled community.