a couple 18+ mature ideas:
a human male/female is kidnapped to be the personal slave/servant of a rather mean seemingly evil king. he bosses them around day and night, treating them like what they were brought to him to be. however, he would grow attached to them, and this would led into other things. During this time, the slave/servant in question was in love with someone before the king got possession of them, but the one in question had since passed away. 






a dark energy vampire in a secret casual relationship with someone who is dragon shifter. There is room for adding to this plot.