yeh, jk has said somwhere that books 6&7 are almost one book in two parts, so that might explain why not many new people will be introduced.
however, book 7 will include harry losing DD, not book 6. i wont start this argument here, im already arguing about it elsewhere.
on topic now, i think that basically the entire of the OotP will reappear, and probably Blais Zabini as well. he was mentioned once, but jk had quite a few notes on him, so i think he will come back. also, i hope for some info about aberforth. what on earth does he do for the OotP? he was in the old one, but moody said that was the only time he ever saw him. what mission does he accomplish? thats what i want to know. i like the idea of the bones family. they lost people to LV, so they might join the OotP as well.
Grawp i think is confirmed by jk, and he is going to be a bit more tolerable. at least, he wont start grabbing hermy any longer, i hope. he'd better not learn how to say hagrid tho. id hav to change my screen name