Sally Hawkins for Trelawny. Patrick Stewart for Dumbledore
Great suggestions. Maybe James Dreyfus could be Fudge instead!
Ben Wishaw for Snape would be good I think - he's an excellent actor and it needs to be someone amazing, and he's young enough. He definitely does nuance and that's needed for Snape.
David Tennant - another great actor but possibly a bit long in the tooth for Snape would be a great Voldemort (you don't need to be a particular age for this role IMO as there will be a lot of makeup). Mind you again, makeup can make actors appear younger, so I wouldn't want to rule him out...
Thandiwe Newton is a bit of an inspired choice for Bellatrix - I like it. Part of me thinks she's simply too pretty but is an excellent actress and presumably the makeup would be fantastic so could be made to look suitably mad. Like Rickman for Snape, following HBC will be hard for anyone though.
Sophie Okendo would be a great Narcissa but I'd still like Tom Felton for Lucius - he did snooty pretty well as Draco and presumably his acting skills have improved since then.
I think I'd like it to be British actors mainly because there is nothing that annoys me more than Americans trying to do a British accent and not quite getting it right.
Kate Winslet (though they'd be lucky to get her) for McGonagall. Although she might be a good Bellatrix too.
Martin Freeman as Arthur Weasley - with a red haired wig obviously!
Nina Wadia might be a good Umbridge.