I will suggest a fandom to you. It is the drama series Vikings. The main female character is the shield maiden and mother, Lagertha. She is in no way a passive or submissive character. Here is a short video of her in action:
m.youtube.com/watch?v=iKdEZUtmRLY
She is married to Ragnar. Here is a video about their relationship:
m.youtube.com/watch?feature=related&v=yl72Ybasghg
They live with an English monk called Athelstan, who starts out as their slave. Here is his relationship with Ragnar:
m.youtube.com/watch?v=HyNz7Mo9n4E
It isn't canon, although Lagertha and Ragnar proposition Athelstan and he refuses due to being a religious virgin celibate monk. It is very appealing to slashers, but because so much of Ragnar's sexuality and romantic behaviour is bound up with Lagertha's character, it mostly has to be written as a relationship with all three of them, but written by people from slash fandoms. It is a small fandom that will hopefully grow (the show has been renewed for a second season). I think that would be an easy way into het because even if you read het bits and think nope, you still have the slash happening in the same fics. The whole thing is immersed in supernatural belief systems in the show too.
If you want something more strictly supernatural, I would look at fic about Erica and Boyd, two beta werewolves from MTV's Teen Wolf series, which is mostly a slash fandom. There are definitely slash writers who also write Erica/Boyd.
I tried to look up fic for you, but am so into angst that I started reading some poignant pregnancy Erica/Boyd fic instead, and so I am probably not the best person to identify your preferred tropes.