The Leaky Cauldron has published a list of characters expected to feature in 'Order of the Phoenix'. The list is not complete, or confirmed by WB and could be subject to change. Neither is it indicative of actors in the roles. It does include some of the regulars whom you'd expect to see, regardless.

Mafalda Hopkirk
Mad-Eye Moody
Dedalus Diggle
Young James / Lily / Snape
Bane
Amelia Bones
Voldemort
Kreacher
Cedric (flashback)
Vernon
Petunia
Malcolm
Piers
Dudley
Mrs Figg
Tonks
Kingsley Shacklebolt
Remus Lupin
McGonagall
Arthur Weasley
Sirius
Mrs Weasley
Cornelius Fudge
Umbridge
Trelawney
Cho
Zacharias Smith
Nigel
Dawlish
Percy
Snape
Magorian
Lucius
Bellatrix
Filch


Source: The Leaky Cauldron

UPDATE:

In addition to the above, we can add the following:

Young Peter Pettigrew
Young Sirius Black
Young Remus Lupin
Fred & George Weasley (yes, their excellent departure from Hogwarts was scripted to appear in the film)

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