Now that the Half Blood Prince is out, we can begin speculating on the last 2 movies, and what the plotlines may be.
I mentioned it in another post, that we can put to rest some of the speculation of what story lines may be followed by looking at the cast for DH (parts 1 and 2). Look here http://www.imdb.com/title/tt0926084/fullcredits (for part 1)
It lists actors/actresses playing the roles of (to name a few) a teenage Dumbledore, Bill Weasley, Fleur, Umbridge, Reg Cattermore, Dobby, Kreacher, Griphook, and even Mundungus.
That does not conclusively tell us everything, but it does give us something to speculate on:
1) We will see a Dumbledore flashback. What exactly and how? No telling, but it may involve Grindelwald
2) The Fleur/Bill plotline is not dead.
3) It is likely that the ministry will be perpetrated, as it was the only scene in the book in which Umbridge (and Cattermore) appeared.
4) With the house elves present, their role will be examined, but to what extent remains to be seen.
5) Griphook is interesting, as if we go down this road, it allows us to speculate where the movies will be divided.
6) Mundungus played 2 main roles in the book. First he was a major part of what happened when Harry left the Dursleys (more on that in a minute). Second, he was the means by which Harry finds the location of the locket (see numbers (2) and (3) above.
7) The absence of any of the 3 Dursleys suggests that their role in bidding Harry goodbye is a dead plot line (no pun intended).
Looking at the DH2 cast suggests other things entirely, but I will let others investigate and speculate on that.