I can’t imagine who they would play. Stuart Townsend and Joseph Fiennes are both pretty young and good looking. Too young to be Scrimgour (sp?) or Slughorn.
Maybe Townsend is going get a dye job and be Bill Weasley?
Joseph Fiennes has a kind of dark, mysterious look to him, so maybe a death eater? I can’t think of any prominent DE in HBP aside from the usual suspects.
I would guess it's Tom Riddle, Sr, as Harry meets him in the pensieve. The added benefit is that you get a true family resemblance to the older Voldemort.