When I first thought about the film, I thought it would be really hard to make, because a lot of it is really emotional and social storylines. But, like you say, the death of Sirius, Umbridge and the battle at the end could make a fantastic film. I think the end of the film will be rushed though (I mean the scene in Dumbledore's office). I think a lot of time will be given to the ministry and the prophecy scene will be ruined by Michael Gambon. This point in the book was a real turning point in Dumbledore and Harry's relationship and I hope they do it justice. Doubt it tho. Plus it's a really dark book and they need to capture that which could be difficult.