Sirius survives the Department of Mysteries fiasco but doesn't tell the majority of the Order of the Phoenix and for the sake of his godson finds the courage to forgive himself for his past mistakes and try to create a better future. Dumbledore and Voldemort had better watch out. Padfoot is back in the game and determined to set things right.
When a prank of Fred and George's goes badly wrong in the summer after the Triwizard Tournament the entire Order of the Phoenix are deaged to toddlers. With nobody else to turn to the twins, Ginny and the Golden Trio have to handle the situation on their own as well as cope with the ever present threat of Voldemort.
After Professor Dumbledore dies Ron and Hermione are obliviated of all knowledge of the horcruxes and for their own safety Harry chooses not to remind them. Finding a way to end Voldemort once and for all on his own won't be easy, though.