Harry Potter is back in time at the end of his third year at Hogwarts. The list of people he trusts is short, and he knows how badly things can go, but he doesn't know where all the Horcruxes are. He's broken, and he's been charged with saving the world.
Harry Potter is a geek, a nerd, a total dork whose favorite hobby is "transfiguring stuff." He doesn't understand what rules are, has no social skills, is barely sentient, and is very smart.
The Hougyoku freaks out, and Ichigo and Rukia are decades in the past, alone, unprepared, depowered, and planning to fix it all, secure in their knowledge of the future. But the past is a slippery fellow.
Rukia figures out a way to get her powers back before being discovered by Soul Society. Lemon at the start, but not a lemon-filled or lemon-focused story.
Seeing as he's her closest living relative who isn't her mother, a Malfoy, or a prisoner of Azkaban, seventh-year student Nymphadora Tonks takes first-year student Harry Potter under her wing.
Creeped out by all the tombstones, Harry and Cedric leave the graveyard almost as soon as they arrive. As result, much is different. Not sure of pairings.
Life is a mystery for Jedi Initiate Obi-Wan Kenobi. His connection to the Force has strengthened, he's become paranoid about the prospect of war, and there's a voice in his head that's undoubtedly his but seems older.