Philip Seymour Hoffman would be awesome as Hakan. Spoilers below:
I don't think they'll show Hakan as Eli's "Former Oskar" , since the people doing the remake said they're going to be more faithful to the book. The book expressly makes him not that way, Hakan had his own complete life before he met Eli, including being a teacher who became disgraced as a pedophile, and THEN he meets Eli. Does make the character a lot darker, and removes any sense of wistfulness or former romance from Eli and Hakan's relationship. Hakan is a pedophile obsessed with Eli, Eli knows that and uses him to her/his advantage. I'm not sure if I like this better or worse than the possibility that Let the Right One In portrayed, but it certainly keeps the focus more on the relationship between Oskar and Eli.
Oh, and I think the original title is actually Let The Right One In in Swedish, the American publishers of the book thought the American reading public wouldn't like that, so they changed it for the translation.