If romance is your genre, stay within your field. You might include a crime, but (a) make it a subplot and (b) it must contribute to the romance. Again, that seems obvious.
Study the work of others who write in the same area. Make sure you understand and consider their writing as your guidelines. But that's not enough: Don't make your book sound like others in the field. This may be subtle, but the differences distinguish the excellent from the mediocre. For example, I can pick up a Christian romance and by the third page I know what it is. It has what I call a twang. The story is usually predictable, but the writing is, one friend said, "too nice to be real life."
Good fiction means to stay within your genre, but not to sound like the others in your field. Say it differently. Think creatively instead of following the formula laid down by half-a-dozen others.
Good writers know their field;
good writers write distinctively.