Of course we are going to see all the monsters return Vampires, Werewolves, etc. And then there are the Alphas.
I personally like to see the Sins return. Maybe a few at a time so we can see the details of each of them. I didn't see nearly enough of sloth and pride for my liking. We already dealed with the parents of them (War= Wrath, Famine=Gluttony, Greed, Lust.)
I'd love to see a return of Missouri, we have not heard anything from her in FOREVER! Also that little Demon/Human hybrid he seemed to be a key piece in the End of times and he just disappeared.
I'd reallylike a return of Jo and Ellen. Maybe not to life because it might belittle the tragic deaths they had. Athough they are not in heaven so they might not be dead, maybe they are in another type of afterlife (possibly Hades, Vahalla, or they could have been reincarnated).
Oh and Kali she was epic!
If it was at all possible I'd wanted Gabriel back, but that is probally very unlikely.