I'd prefer it if it happened to one of the protagonists, and if the problem was stretched over the course of the manga. Realistic relationships are a plus, as well as dark ambiances.
It would also be nice if the two characters got together in the end, but that isn't required.
Thanks for any help.
Some examples:
Nocturne (Park Eun-Ah) - Age gap, guardian-child relationship
Spoiler (mouse over to view)
Tale of Felluah - different social class
Hirunaka no Ryuusei - Age gap, student-teacher relationship
A method to make the world gentle - Age gap, student-teacher relationship