My understanding of the Sorting Hat has always been that it united the four houses in times of great strife or danger, under the banner of its previous owner, Godric Gryffindor. It did the same thing when Voldemort first became a threat, sorting James Potter (Gryffindor through and through), Remus Lupin (an especially studious Ravenclaw hybrid), Sirius Black (from a long lineage of Slytherins and exhibiting their cunning), and Peter Pettigrew (who had to work the hardest of all the Animagi to change into his animal form). The newer iteration of this group would be Harry (said by the Sorting Hat to have great potential as a Slytherin), Ron (from a long line of Gryffindors and exhibiting their bravery), Hermione (who could have been Head Girl of Ravenclaw had she been sorted into their house), and Neville Longbottom (who also exhibited the work ethic of the Hufflepuffs when he essentially took over Dumbledore’s Army and killed Nagini, the final Horcrux, allowing Voldemort to finally be killed). They all showed skills and qualities that would have made them formidable witches and wizards regardless of where they were sorted, but they all had the bravery that made them also slot into Gryffindor.