It's kind of difficult to answer. Right now the background only exists as a vision in Jim Sachs' head. Expect it to look spectacular though.
The creature pack is equally difficult to predict. We've snapped up various bits and pieces of information over time, but I don't think it's decide which creatures will end up in the pack(s).
Here is a link to the Official Wishlist's (not the wishlist thread) section for speciality creatures.
Personally, I suspect an anemone with a Clownfish living in it will be part of the first pack. I know Jim's a bit annoyed by the free-swimming Percula Clownfish in the current aquarium.