I'd like to see hints about great warriors that uprooted kingdoms with their strength (korgan), very powerful awe-inspiring drow (viconia), elminster having a great battle with an uber red wizard (edwin) or stuff like that.
If the story IS before BG1 there will be (again) a godlike storyline (killing the lord of murder) and if it's in the FR backround (probabily a lot of time after BG) it would be cool to find Minsc's Sword of A-lot-of-damage, Robe of Edwin, stuff like that. Make us remember the epic storyline that unfolded with BG!
But I'd sure love to find my party (korgan, viconia & edwin)
you can't kill the Lord of Murder, the mortal Cyric (an evil fighter/mage from Calimport) killed him in a temple to bhaal himself on the eighth day of Eleasias the year 1358 DR, the year of shadows and the year of the Time of Troubles. Cyric and Bhaal were both competing over gaining Bane's god portfolio when Bane fell to Torm.
But that's offtopic, the NPCs I would like to see in BGIII are
prequel: Gorion, Minsc as a child(he is older than the PC, but how much older?), Elminster would definitely be possible, he's almost 600 years old, and Volo is cool
sequel:some cameo appearances of the NPCs from BG1+2, meeting the Justice Fist(Minsc's new party) would be cool, but other than that all new NPCs
My two coppers.