I struggle with "too dark" also.
My novel is meant to be a Lord of the Rings for a new generation, assuming we've matured as a society enough to accept some very violent and even taboo scenes. But none of the great classics seem to include any of the most foul pits of the human psyche. Therefore I sometimes think my work can't become mainstream. (too dark)
I envision it as a movie someday. Thus, I've changed an unnecessary disemboweling scene to his femoral arteries being cut.
However, the attempted rape and subsequent cannibalism I feel is an important plot point. Neither are explicit in their descriptions but I feel it would probably earn the movie an R Rating, and keep it out of high schools and under for reading. A child dies in very graphic way. It is meant to jar the sensibilities. The theme is largely about death, which is very sad and awful until the characters learn the truth, and can think of it in a different way.
So I don't know. Can one attain Harry Potter popularity while including some very adult themes and scenes?
Please don't take this the wrong way, but including wall-to-wall rapes, disembowelling, and cannibalism does not inherently make a work more adult. Rape in particular is a very sensitive area, with substantial psychological issues associated with it - it should not simply be used for shock value.
Also, what, exactly, do you mean by "great classics"? Taking The Lord of the Rings as a starting point, you have catapaulted heads, an on-screen suicide, a cannibal major character, and a swamp full of corpse-ghosts. The Silmarillion is darker, with nearly everyone dying, multiple on-screen suicides, incest, allusions to rape, and so on. A Song of Ice and Fire and The Chronicles of Thomas Covenant are pretty dark as well, and if you are into more recent stuff, there's the likes of Joe Abercrombie and Richard Morgan.