That thing where every British actor has already been in a Harry Potter movie isn't exactly true. Here's who should be put in the next flick according to me, Supreme Mugwump:

Rufus Scrimgeour: Derek Jacobi

Narcissa Malfoy: Tilda Swinton (despite Narnia conflict)

Horace Slughorn: Brian Blessed

Merope Gaunt: I would have said Shirley Henderson, but they've already used her. I'm going to go to the bullpen and grab Kelly MacDonald, not least because I enjoy looking at her. Kate Beckinsale wouldn't suck either.

Morfin Gaunt: Jason Flemyng

Marvolo Gaunt: Ian Holm (conflict again, but why not?)

Tom Riddle Sr.: Toby Stephens

Fenrir Greyback: Bill Nighy

Regulus Black: Only one person alive can play Gary Oldman's badass kid brother, and that man is Johnny Depp.