I don't want to see the current aquatic pack animals replaced. I love the gray seals, and I think the otters are cute (though CA sea otters and sea lions would be awesome additions to a future expansion).
Someone mentioned puffins up thread, but those could fly away in the current exhibit system. I'd definitely like to see an expansion or two dedicated to smaller, flying birds that includes the possibility of building aviaries. I suppose they could easily create smaller "exhibit" type bird displays, with cages instead of glass cubes. In addition, what would be really cool would be walk-through aviary habitats (enclosed by a sort of mesh or glass, depending on clime, so the birds can fly around but not leave), which are always popular at real-life zoos. You can already create something like this with peacocks or flamingos by putting a walk-through enclosure inside a glass dome (though the latter always stress big time if people are nearby), but it would be cool to see smaller bird species like parrots, toucans, cocks of the rock, hornbills, raptors and so on. I suppose a three-dimensional flying aviary would be a need to create some kind of three dimensional glass or "null" barrier to do this, however.