As part of the OOP movie press junket (see TLC's full report here), director David Yates said that in the original draft of the screenplay for OOP, they had decided to remove a character from the story. Rowling read over the screenplay and said “I wouldn’t do that if I were you,” that they were free to do whatever they liked, of course, but “if you make a 7th movie you’ll have tied your hands.”
According to Quint (who makes a full report at the link above), Heyman wouldn’t reveal who that character was but Quint and others speculated it could be Grawp, Luna or Bellatrix.
I think it's probably Grawp, just because that would otherwise be a really easy storyline to cut as it's really not related to the main "Dumbledore's Army" part of the plot. Luna and Bella are both a little more essential (Luna with the DA and the thestrals; Bella with the killing Sirius*). Also perhaps Aberforth Dumbledore (who, if you'll recall, makes an appearance as the bartender at the Hog's Head) or Mundungus Fletcher.
(*It may be a little extremist of me, but I refuse to treat this as a spoiler as many sites seem to be doing. It's been 4 years! If you don't know by now that Sirius is dead, then you obviously don't care enough to warrant further spoiler protection. Sorry, that's my rant of the day....)