i already said why they shouldnt bring in angels and demons. Kitsune should only be brought if, lets say Silas is a kitsune. Other than that, they shouldnt come in. Other than made for tv series characters, you could have all the living supernaturals return-lee, charlotte, abby, nandi, lucy, and galen vaughn. Also bring in the book characters:

Caleb Lockwood

Zander and his pack

Sue Carson and Cristian Fell as human characters

the vampire characters of Ethan, his european lady maker and Jessalyn

the book character Mrs. Flowers has been shown, but she could be upgraded

I also feel in order to fit the pattern, if Silas is defeated, perhaps they need to bring in more witches to be the final villians, try to bring Bonnie to their cause of eliminating supernaturals from the planet