We need more diversity in western visual novels. And I say this admitting that I like moe-style romance games...
If you look around a bit, you can find some western VNs with distinctive art and themes. This seems to be Bento Smile's specialty -- the person is involved in the general indie games scene, and makes a number of unusual and deconstructionist stories. Likewise, while Christine Love's games tend to involve relationships, some of them have unique art (Digital: A Love Story is presented through an Amiga interface, Heart of Fire is presented through 8-bit sprites), and they usually feature realistic gay characters. There's a number of others which I'm probably forgetting.
Right now, the scene is fairly small. And if you want to *sell* an independent visual novel which lacks fictional relationships or a current anime/manga-esque art style, then it might be difficult... Jack_Norton has tried with the first two Vera Blanc games, but they didn't find much of an audience. Moacube is currently working on Cinders, which will hopefully be finished in the near future. Other people mentioned the commercial game Hotel Dusk. I don't know how it sold... its follow-up game Last Window did have an English version, but was only sold in the UK for some reason.
Also, here's a little secret: some of the first visual novels were serious mystery stories. If you haven't heard of
The Portopia Serial Murder Case, then look into it. There's even some English patches available for the Famicom version. Likewise, there's a blurb on the Visual Novel Database which talks about how Chunsoft's vintage games use art which one might not expect -- pixel art, photographs, and even live stills.
In any media form, it helps to look at the history, so that you can see the not-so-commercial products that came before the current stuff that's selling squillions. If you haven't seen a non-superhero comic in a while, read Understanding Comics. If you can't remember an eastern RPG which had any adult protagonists, then look back at Final Fantasy 4, early Suikoden, or the first Shin Megami Tensei. If you look back on the history of comics which appeared in Weekly Shonen Jump, then you'll find ones which might seem shocking now, including Fist of the North Star, City Hunter, and Kazushi Hagiwara's series Bastard!
One more thing... I'm quite aware of the Animation Age Ghetto. Western cartoons have often been ignored, and treated as ephemera at best. It's taken many years to get more than one cartoon on prime time television. So yeah, we don't need a '90s style grimdark cartoon just to shake these things up. And while some of the stuff that Manga Entertainment sells is a good alternative to the current moe/harem/slice of life shows, storytelling is much better than shock value.
So in short, write what you want, and use whatever art you want. Good luck.