I very much enjoyed the substitute problem that you tackled here. I'm a bit surprised that not more people use it. What I would consider the perfect post-DH fic would be something where Snape gets together with Harry because he sees Lily in him or tries to replace her with Harry and then gradually learns to love Harry in his own right.
Most people just avoid this whole issue by reducing Snape's relationship with Lily as purely platonic. I like that, too, but I also want to see the other approach done.
So, your fic is kind of the first step in that direction (along with Conjure-Man by Ishafel) and now I'd like somebody to do the next step. ;D