I can't see tonight's unseen footage being more awful behaviour from Bec or Gia. They've been the central storyline of the whole series so I would have thought they've shown all of their worst moments already.
I reckon it will be either something about Danny - evidence of him stringing Bec along or flirting with someone else.
OR it could be something to do with Alissa. They've largely ignored her in favour of the Bec/Gia feud, so there may well be something that incriminates her and vindicates David.