Fleur, Harry, Ronald, Draco, Luna, and Minerva all fit in his Headmaster's Office. It was agreed that Severus would stay as Headmaster with Minerva as Deputy and the Transfiguration Professor. Slughorn was goaded into staying for another year as the Potions Professor, and there was a near consensus on who would teach DADA. Of course it'll either look horrible or brilliant, and I guess this is how Albus always did it.