These games are very similar in structure to the old Choose Your Own Adventure books where the plot changes and follows a different path based on the reader's choices.
The visual novel Bionic Heart (2009, Mac, Win, Linux) by Winter Wolves is set in 2099 London and offers "bionic beings, complex technologies, relationships and romance." It is fully voiced and has 24 different endings.
Instead, having very good writing skills is far more important for both visual novels and dating sims (although a bit more important for the visual novels).
Writing skills also means coming up with original plot ideas and carefully planning each path.
As for theme/setting, anything goes: from dark sci-fi to cyberpunk, from historical to light comedy, like my latest dating sim "Roommates".That does not mean, however, that you need to have countless images.My first visual novel, Heileen, was made of 15 different painted backgrounds and 16 characters, each one with different outfits and expressions.To be clear, they originated in Japan and, while many visual novels or dating sims are erotic (or at least sexy) in nature, it's not true that all dating sims have erotic content.It's really a shame that people automatically label these games as "adult only" because, in many cases, is not true.