1. Witches/Magic
2. Mermaids with underwater diving, swimming and exploration mechanics, for normal sims as well. A tropical vacation pack much like Island Paradise.
3. Werewolves
4. University
In this order, basically. I am pretty sure at this point we are guaranteed to get Witches and University, just a matter of time. Though I highly suspect the next EP and GP will be University and Witches. There are other things I'd also like to see but they are nowhere near the above.