Hoping we finally get the long-awaited horse pack. I understand horses are hard to animate, but they've put horses off twice--originally, they were apparently planned for Cats and Dogs, and were cut. Then, Country Living they were cut from as well. The supposed reasoning was because they wanted to have a pack focusing solely on horses in order to do them justice. However, putting them off for 6 years is a little much, given we're entering the end of the lifecycle for TS4 before TS5 comes out.
To me, in order to really do them justice, it will need to be an expansion pack. There are a lot of CAS and gameplay assets needed, as well as probably what would end up being the largest world ever in TS4--and comprised of very large lots.