This story will loosely follow canon and split completely at the end of Year Four. The primary focus will be the Horcrux in Harry's head and the use of ancient magic passed on to Harry thru 'incarnations'. Much of the story will reference elements of FASA's Earthdawn game.
The story will diverge from Canon in that the Horcrux in Harry's head affects others around Harry - just like other Horcruxes affect others in near proximity. His mother's sacrifice protects HIM, but others feel the effects of the Horcrux. This explains the many times the school seemed to turn against him for very little reason. It also goes to explain why adults tend to look the other way so frequently. All Harry knows is that people tend to let him down. In this story, the negative moments will be a worse.
An 'Incarnation' is from the Earthdawn game where a Light bearer (people who protect the world from evil entities) can pass on much of their knowledge and talents to a recipient. The 'greater incarnation' actually allows the person passing the knowledge on to serve as a "ghost master" or spirit trainer. To me, this is kind of like a 'good' version of a Horcrux. It would be like having a wizarding painting in your head that you can learn from. Easy way to 'level up' or just learn faster. Imagine having a tutor available 24/7. Even while you sleep.
I will introduce a few new branches of ancient magic from FASA's Earthdawn game. Elemental, Illusion, and Nethermantic magic. They are not more powerful than modern Magic's, but give a new dimension and spells that are not standard. They are in many ways more ritualistic and less flexible than modern spells, but easier to cast wandlessly as they pre-date the use of wands. A different take.
I like the powerful Harry stories and this is my way of accelerating his learning. He still has to apply himself, but the knowledge is now far more accessible. In this story, it answers the question of how Harry was able to become a Horcrux – Lily and James' ritual was used to prepare Harry to receive their Incarnations. Two incarnations to be specific. Since Voldemort had prepared a darker and twisted version on himself, the split part of his soul had a place to land, Harry's 'second incarnation' spot.
The incarnation is a form of 'Sacrifice' magic, powered by the willing sacrifice of Harry's parents. It goes hand in hand with the protection Lily set up. A Horcrux is a form of 'Death magic'. Death magic is similar to blood magic, but more powerful. It comes from the murder of an unwilling victim.
Voluntary 'Sacrifice magic' is more powerful than Blood or Death magic and that allowed Lily's incarnation to partially contain the Horcrux. Her incarnation and protections prevented Harry from succumbing to its effects and containing or 'Trapping' it in the scar. It may be trapped, but it is not a willing resident. It will constantly attempt to assert itself – affecting those around Harry.
Note that I call the part of Lily left behind as her incarnation, not her soul piece. It is a reflection of who she was when she died. With all her knowledge and personality. Her soul (and James' soul as well) has moved on, leaving behind the incarnation to watch over and guide her child. Again, think wizard painting.