If anything, Snape is a deliberate inversion of Heathcliff. So I suppose in that sense, JKR might have had Heathcliff in the back of her mind.

That's what I would say, yes. After all some think (Harry, especially) that Snape is out to destroy Harry's life, and that he is an sadist prick without redeeming qualities. It's just the twist at the end(with hints before) that he is different. As if she wrote a version of Heathcliff that is redeemed, a Heathcliff who was steered in the right direction by love.

Lily seems similarly opinionated, proud and wild as Catherine, also naturally attractive and popular. Like Cathy she ruins her relationship with her older sibling for her friendship with the weird outsider, and later gives up that friendship for a healthier one with people whom she can imagine a better future with(neither Cathy nor Lily make these decisions consciously, at least not the giving up of one for the other part, it's just what happens because they do what they want and because of their and Hindley/Heathcliff/Tuney/Sev's character weaknesses + maybe the Lintons/Marauders rude behavior and social differences). Very unlike Cathy she dies for her baby, instead of dying because she doesn't care about her baby. Unlike Cathy she is happy and healthy and future orientated. She's like an inversion of Cathy. IMO

LOL I love this topic, if you can't tell.