Set your story in the Aztec Empire, which went from one of the most dominant empires in human history to nothingness in a few years. When Cortez landed on the shores of Mexico, the Aztec Empire was at his zenith. EDIT: If you are looking for a slow decline, you have the Maya Empire centered on the Yucatan Peninsula.
Between 730 and 790 AD, the Maya Empire reached its greatest growth. However by 900 AD, the Maya Empire had collapsed, leaving a collection of city states along the Yucatan coast. These city states lasted through the Spanish conquest in the 16th century, giving you a slow collapse of an empire -- about 100 years -- and then residual glory of the empire in city states with some leaders perhaps wanting to restore what was.
You also have in the collapse of the Maya Empire a "perfect storm" of reasons for the empire to collapse. And because historians and archeologists don't know a lot about the civilization, unlike Roman or Greek empires, you have a lot of room to work with here in a fictional setting. Among the elements that led to the Mayan collapse are invasion by a nearby people, the Toltecs, changing trade routes that took the very lucrative cacao trade away from the inland Mayan cities (much like sea routes took the silk trade way from the inland silk road, collapsing the economies of cities built on that trade); a 200-year drought caused by the Little Ice Age; rebellions by the people against the priests and nobles when the rain stopped and the crops stopped growing; political turmoil among the rulers; and a systemic ecological collapse caused by the Mayans slash-and-burn farming techniques combined with the drought.
So you have a whole lot of things going on here -- pressure on the borders, economic pressure caused by lessened trade, religious pressure caused when the gods stopped the rainfall, food pressure caused by declining crops, rebellion in the streets among the people and political pressure caused by priests and nobles jockeying for position in a declining empire. You can follow a priestly and/or a noble family from the empire's start through expansion to its greatest eighth to a slow decline through the 9th and 10th centuries to city-state status to attempts at supremacy in the region of the city state through the Spanish invasion to the end of the city state with the Spanish conquest. In all, you could probably write a fictional history of over 1,000 years (5/600 AD to 1600 AD) by concentrating on the Maya Empire.
And, because there are few Mayan scholars -- at least in the public and unlike the Greek and Roman empires -- you would have few critics. And, since most people have heard of the Maya Empire and a lot have visited the ruins in the Yucatan, you would have a familiar setting.
Adrianople, AD378. The annihilation of the Roman army of the East marked the beginning of the end for the Empire. A hundred years later, Rome was being sacked.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.