It’s great to hear that you’ve enjoying the unique monsters!
There has been a vengeful spirit in the First Interlude story, it was simply glossed over so the short story could focus on their bonding. Plus we’ve got the demon and Celeste the witch hanging around on the outskirts.
However, there are going to be more spirits coming up as well, along with plans for vampires and maybe a werewolf on the periphery. Remember, this includes the AUs as well, so there’s no way of knowing what will turn up when, or who will turn up. Other Supernatural characters may walk their way in at any point in time.