First off, Slytherins aren't evil.
And I hope Harry doesn't get Head Boy, but I doubt it will happen. If Lucius is still in Azkaban, then it probably won't be Draco either (He'll get out, with the dementors siding with Lord V, but when Lucius escapes with their aid, is anyones guess...)
Hermione will undoubtly be Head Girl - I reckon we'll probably see some unknown Ravenclaw take up the post.
Or, like lilmsqt said - it could be Ron. While I dont think the mirror of Erised shows the future - just like Albus said, it shows your deepest desire, which for Harry, is to have a family of sorts - it could be an indication that Harry will die and be reunited with all his dead relos. Or not, just a thought.
As for the backstory to Hogwarts, I'd LOVE that. They did it with LotR, why not HP? 12 books on the History of Middle Earth, written as a reference book rather than a narrative, it would be wonderful! Even if it was written like the index in the LotR, it would be grand! Well, I'd buy it. There is no good reference books with HP, whereas Lord of the Rings has an ambundance of them, and they really help shape the world created. Hogwarts: A History would be great. well, I'd buy it at the very least...