I assume the very next one would be some stuff pack. After that maybe witches GP.
It's too soon for next EP, and the beaches could be not recived very well now as it's potentially gonna have some summer activity people were expecting in Seasons. I feel that the Farm GP is on rather early stage of development (if it is at all), so it leaves the supernatural packs, and I got impression that the idea of witches has more fans than werewolves.
Edit: Now that I think about it, it could be the pools stuff pack. I know it's kinda the opossite of what I said (about thing that should've been included in Seasons), but SP is smaller than GP and maybe more players could afford it. Also, it fits the real life summer, and they'd have not so bad explanation about deliver the pool fun without the necessity of buying the whole EP.