Who loves ya, baby?
Great post, but you forgot 5) - the invading barbarians, which were real to the late Romans, of course, and imaginary for the modern USA and Western Europe, although no less threatening to the populist imagination for all that. (And to the extent that both regions are subject to immigration from cultural outsiders, these people are often fleeing economic and political conditions for which Western countries are at least partially culpable - then again, maybe you could say the same about Rome and all those Vandals and Anglo-Saxons and so on, I don't know.)1) small agrarian republic largely based on a slave economy, with extremely high value placed on personal honor and public service as function of that personal honor
2) becomes a world-spanning empire, defeating its mortal enemy in the process
3) only to find itself essentially adrift as the world's (the known world, for the Romans) only remaining superpower
4) finds itself engaged in imperial overreach, its traditions long ossified into corruption, wealth inequality yawning, might turned inward without a great external enemy to unite its people, the normalization of public violence, etc