Bonus if there's a plot other than the romance, like if they're Adventurers a Quest or learning at a magic school or fighting demons or something.
Something that is both unrelated to and strengthened by their relationship.
Like the book series Fates parallel is a great example because in the beginning it's about two girls going to a magical martial arts school and ending up as roommates, but they eventually like start linking their souls together and their relationship literally becomes their fighting style.
Something like that would be perfect.
by okidonthaveone