Let's play 'what-if'. Take a instance in any of the four Harry Potter books, whether it be something mentioned from the past, something during the books, or whatever, and create a 'what if this happened instead'. For example: what if James Potter had been bitten by a werewolf instead of Remus Lupin? Then ask yourself some questions like would Lily have fallen in love with him anyway? Would he have inspired the other three boys at school to become animagus and if so, would their animal forms have been different?
Take any instance but it must be something mentioned in the 'canon four' books. Do not hypothesize on something that hasn't happened yet? (like what if Severus is a double-agent against Dumbledore, or something along those lines).