6. Animagus Trouble.
What if Harry (or someone else) finds a spell (could be from the founders but thats a bit overrated no? I could make up a name...) to evolve an animagus so that the witch or wizard has, when in human form the animagus form abilities. For example if you are a wolf animagus, you would have, enhanced eyesight, nightvision, reflecting eyes (like an animal), enhanced strength, etc.
But what if you got all the instincts that come with that?
Harry (or another male character) and a female (for example Hermione, Blaise or Ginny) share the same kind of animagus form (for example a feline or a canine), the female goes in heat and only one of the same kind can quench it. This also means that the one who quenches it is her mate. And all animals have strong instincts concerning their mates….
Or something like Harry has a wolf as an animagus form, he becomes VERY protective of his 'pack', doesn't respond well at all against being challenged, demands a submissive attitude from anyone 'beneath' him (pack hierarchy), the other students sensing something 'off' subconsciously, act submissive/challenging toward Harry (Draco challenging Harry for dominance, Harry wins, Draco acts submissive from that moment on and becomes part of the 'pack' or leaves Harry alone from that moment on). This also includes psychical changes of course. Harry could be a magical kind of wolf and controls that magic in his 'human' form, Harry having feral eyes in some situations, really sharp nails, that sort of thing.
This could be done with another animal as well of course (feline? bird? Another canine? Bat? Dragon? –oooh that could be cool! You could set most of the instincts and reactions yourself since there isn't a lot known about dragons- other magical creature?)