Re:Birthday Song has recently gotten an english patch, and the premise seems interesting enough so I say, you ought to give it a try! I'm attempting to get the game to work here myself, though the game crashes from time to time and there's no proper fix yet (probably because of the patch itself; it's the translator's first work, after all).
I could say Hakuouki
would also be good, but what exactly do you consider 18+? In one of the routes, there's sexual implications, and then there's also a lot of blood here and there (the game IS set up in some sort of fantasy Bakumatsu era, with samurais and all that jazz). But overall, it's a good game with good characters, and there's lots of content since Idea Factory keeps milking off of it (not that it's anything bad though, haha. There's even a new game with new content coming out this year, I think, for VITA).
Also! There's Ayakashi Gohan, and even though only the DEMO is available for free+Fan Translation, it seems to be really interesting! I played through the demo, and the heroine is not only relatable but also one of the best I've seen around (I tend to see the heroine as her own character rather than an insert for the player).
Then, though I didn't personally enjoy them very much, there's also Amnesia and Ozmafia (they both have fantasy elements, though since I didn't finish them I'm not sure whether they have 18+ content or not. I'd definetely keep away from Ozmafia though, I played only through one route and it was quite the rollercoaster. Heroine has no character, and narration itself is in third person, if I remember correctly, which I think not many people might enjoy. It's your decision though, so if you wanna give it a try I ain't gonna stop you.)
And for now, I guess that's it. A game that seems to have a fantasy aspect I'm waiting for is Nightshade, though again, I don't know if it's got 18+ content as in, nasty couple session, but the premise is also interesting enough. I guess that's it for now, and I hope you find something to play soon enough! Happy gaming!! =)
EDIT: ALSO!! If you're into general games, as in, let's say, JRPG, Persona 3 Portable allows you to play as a girl. Basically, you summon aspects of your personality that take form of certain deities to battle against monsters during nighttime, all while managing a high school life. You can date three of your party members + a mystery boy, though one of them makes me a bit uncomfortable and the other... Well, you'll only know if you play.
