Enchantress2 said:
I like the idea of Jack the Ripper, in fact, they make a joke about him in the movie, but I think he might try to kill Mina.
And I didn't recognize reading about Dorian Gray from a book. Which one is he in?
Jack - yeah, try would be the operative word. He usually plied his trade on old, drunken prostitutes, not vampires.

Of course, one could argue he already *was* in the movie, since I've heard that "Dr. Jekyll and Mr. Hyde" may have been inspired by Jack's exploits. And to make the joke a bit more ironic (at least to me), the East London Docks, where they were at the time, was pretty close to Jack's stomping grounds.
Dorian was in "The Picture of Dorian Gray", strangely enough.

Dunno if it would help at all, but the author of that story, Oscar Wilde, appears briefly in From Hell, another comic by Moore.
I concur with the guy who mentioned it earlier, Aleister Crowley could be neat as a villain.
My suggestions:
- Springheel Jack, apparently some disturbed individual who liked to jump around London and scare people. Later, he became a somewhat comic book type figure in penny dreadfuls.
- Inspector Legrasse, from HPL's "Call of Cthulhu". I think the Louisiana bayou section of the story dates back several years from the rest of the story.
- Dr. Abel Trelawny from another Stoker novel, "The Jewel of Seven Stars". An adventuring Egyptologist.
- The doctor (what was his name?) who invented the Food of the Gods in H.G. Wells' novel of the same name. Alternately, one of the giant children from the novel, maybe as some sort of bruiser-type.
- The Elephant Man. Don't laugh, they could play off Moore's suggestion in From Hell that he was a sort of oracle, an incarnation of the Indian god Ganesha as Dr. Gull suggested. He wouldn't be the adventuring type, maybe a guy who hangs around at the base and offers advice occasionally. BTW, Gull's line to Mr. Merrick in that book, "Make certain you don't do anything sudden in your sleep," was oddly prophetic, now wasn't it?
- I think it's going back too far in history, but Mad King Ludwig would be interesting.
- A villain could be Mary/Helen (which was her name?) from Arthur Machen's "Great God Pan".
- John Silence, the occult detective from Algernon Blackwood's stories.