** Contains Spoilers**
I have the book which I have yet to read. So this "theory" is based on my one viewing of the movie...which means I need to see it again. But I just felt like throwing it out there.
The film concluded with Oskar traveling in a train, and Eli in a trunk. Which pretty much meant that Oskar left home to be with Eli, that he has become her new companion, maybe provider? So is this how Håkan started out? Once also a child, a playmate of Eli. He grows up and Håkan's role in their relationship changes with age but he is still ever so dedicated to her, providing for her and ended up dying to protect her. It sure does look like the path Oskar is going down. What do you think?