One of my favorite pairings to RP out is one with a young man/ Older woman relationship. Here is a few ideas for that.
She is his mothers best friend.
She is his boss at work.
If you can think up any other ways that they knew each other that would be fine. I like to flesh out a story on a case by case basis. There is another pairing I would like to try out and it would be a man/vampire seductress. It could start out that she just wants to feed from him and keep him around as a source of blood because she wants to try out not killing a man for once and see if she can keep one around as a source of blood and perhaps as the story goes she falls in love with this mortal and ultimately offers him immortality to live forever by her side or something like that.
There are a few other pairings that I would not mind doing as well.
Older Man/Younger woman