Oh and another thing- Oda counters the Dragonball syndrome of villains that are "the last guy, but even stronger" by actually seeding characters and factions years in advance. It was I think more than a decade from Doflamingo's introduction at the end of Alabasta to the point where they actually fight him.
Big Mom was introduced, conceptually, as one of the Four Emperors and on the same tier as Shanks and Whitebeard, years before they ran into her.
The Marine Admirals will be a recurring thing probably until the end of the series.
Very little comes out of nowhere just in time to fight it.