Adventurers - Light Fantasy / Medieval 
Knight x Princess - Light Fantasy / Medieval 
Royalty x Servant - Light Fantasy / Medieval 
Princess x Prince/Princess - Light Fantasy / Medieval 

College Students - Modern
Student x Teacher - Modern
Foreign Exchange Student x American Student - Modern / Possible Light Fantasy
Stepfather x Stepdaughter - Modern
Stepbrother x Stepsister - Modern
Celebrity x Fan - Modern / Possible Light Fantasy

Gifted Students - Light Fantasy / Modern
Angel x Demon - Light Fantasy / Supernatural / Possible Modern

Some ideas of mine:

Doctor x Patient
Doctor x Nurse
Mother's boyfriend x Daughter
Father's best friend x Daughter
Online friend x Online friend who could be friends in real life, strangers or enemies

I would prefer to double as well. I'm preferable with MxF but may be swayed to do MxM or FxF if it's not all about "taboo" and that the characters matter, not just the gender of them.