Oscar Wilde was a Janeite.The Ho Yay-packed The Picture of Dorian Gray is basically a Gender Flipped version of the Les Yay-packed Emma. Emma even paints Harriet's portrait!
Dorian is a wizard and doesn't know it.When he murdered Sybil Vane (albeit indirectly), he unconsciously turned his portrait (an object he'd invested a good deal of his soul in) into a Horcrux.
Lord Henry is a Slaaneshi Cultist.Think about it, he corrupts an open mind to become a hedonistic monstrosity, and there was clearly someone listening when Dorian made his wish that the portrait would age instead of himself. I'm willing to bet there was a Mark of Slaanesh hidden on that portrait somewhere.
Dorian's Deal with the Devil was acknowledged and accepted, but it didn't actually take effect until it got to the point where he would have started showing visible signs of ageWhen Dorian impulsively says that he'd sell his soul for his painted figure to age and for himself to have eternal youth and beauty, it's not like he becomes The Soulless immediately afterward. For a while afterward, he is going to see the Shakespeare plays that Sybil Vane performs in and falls in love with her to the point where he has promised to marry her and, from what he says of her, she has become his Morality Pet. During her last performance that he sees, though, they have both found true love for each other. So obviously, at that point, he had still not lost his soul. Undoubtedly, since he was falling more deeply in love with Sybil, he was smiling at least almost every time he was around her. He was probably smiling so much that, had he not made his Deal with the Devil, it would have made at least a slight wrinkle or wrinkles somewhere on Dorian's face that would not be immediately noticeable, but visible to anyone who really took the trouble to notice. Since he made that Deal with the Devil, though, he lost his soul at the very point where he would have started wrinkling and changing in appearance from age. In exchange, he didn't start wrinkling, but the portrait did, and the only reason he didn't notice was because he was so callous toward Sybil Vane that his cruelty was reflected in a smirk that was more noticeable than a slight wrinkle.
Dorian Gray is inspired by Jack Harkness.Oscar Wilde met him twice, the second time much later than the first, and noticed that he hadn't changed. He came up with an explanation for it and wrote it down. Because there's no way Jack has never met Oscar Wilde. (And there weren't any Doctor Who related Wild Mass Guesses here. Everything needs at least one).
Dorian was a rare experiment by an Incubator in making a male ContracteeHe got his wish, but never truly despaired until the very end, the painting serving as his Soul Gem.
The paint used for the portrait contained the unknown impurity that made Jekyll's potion effectiveThe potion changed the drinker's appearance (and mind) to reflect the evil part of their soul. The painting changes appearance to reflect the state of the subject's soul (and arguably has an effect on his mind by enabling him to sin and still look respectable). Also, the moment the evil acts show, Dorian begins to "loathe" it without really being able to put a finger on the cause, sort of like Hyde's undefined evil looks.