Huh, this is interesting. :D

For the most part, I agree with Furienna; Harry wouldn't have been Harry should he have been sorted into Slytherin. It would have been a huge chain reaction; Harry would definitely have become dark, thus making Voldemort not too keen to kill anymore, I think - he'd probably become his heir somewhere down the line, especially if Snape tells him about Harry. Snape himself? He probably would have been so shocked, and thus, relenting on Harry. Perhaps would have grown to genuinely like him. However, I think it would have taken a long, long effort on BOTH parts for this to work, just like it would have for Gryffindor.