Okay, um... here's my idea of a plotline.
The game is set right at the start of the series, but rather than the girls being teenagers, they're adults (or close to that anyway). The original Queen Serenity was strong enough to keep Beryl from acting for longer, so that the Dark Kingdom wasn't able to start its activities until much later. Additionally, other inhabitants from the Moon Kingdom have started to awaken because Luna and Artemis aren't the only survivors.
Also, most of the monsters are going to be different, more hentai-intended. Don't expect the worst right at the start, though.