Duh! I completely forgot the bit about the HBP!
Okay, so in my scenario the HBP is Hagrid. We know that someone learns magic late in life and so that fits too. Hagrid is going to be very powerful, and vital in the fight against Evil. I think I remember reading that it takes a lot of strong magic to work on a giant, or something to that effect. We've never seen anyone actually use magic against him, so we don't know the actual extent of that. So, Hagrid regains the right to practice magic, becomes very powerful and has a great defense (much like Lily's love for Harry, it is Hagrid's giant mother's blood running through him). He could possibly be a weapon against the Death Eaters (but I think Harry will be left to fight Voldemort himself).
As for how this fits in...Grawp will have to become more civilized and perhaps tell "Hermy" and Harry about how his mother was a Queen and perhaps something of Hagrid being the eldest son, and therefore the Prince. But in order for that to mean anything there is going to have to be a prophecy of some sorts or something to make that important. Trelawney is still at the school, so maybe she could do something, there must be a reason why JKR made Dumbledore keep her at Hogwarts...So perhaps Trelawney prophesizes that a HBP will be vital in the defeat of the Dark Lord and Dumbledore thinks it is a good idea for Hagrid to come back to school, as a student. Of course Malfoy will think this is hilarious, as he has always had it in for Hagrid. So he will send an owl to his mother or father (Death Eater connections) and Voldemort will find out that they have a weapon, hence the distraction! Then Hagrid will be w/ the Order cleaning up the distraction while Harry is pushing away his friends and then Hermione will figure it out tell Lupin or someone in the Order, and all will rush to Harry's aid imprisoning more Death Eaters and defeating Lord Voldemort once again...