So, thinking on my own question, I think the answer somewhat depends on the question of why Rome doesn't fall. It helps set the basis we are working from.
One possibility - Rome doesn't fall, because it doesn't become the Imperial Rome we know. Perhaps Julius never crosses the Rubicon. Or maybe, when he's assassinated, the optimates' view held - they killed Julius Caesar, Mark Antony, and several of the others, leading to a shaky but successful return to the republican form - we have the Roman Imperial Republic...
But nobody wants to model the Republic. They want to model the Empire with a Ceasar. So, let us consider the more common points of failure.
To quote a common text of Wikipedia, "The Roman Empire lost the strengths that had allowed it to exercise effective control over its Western provinces; modern historians posit factors including the effectiveness and numbers of the army, the health and numbers of the Roman population, the strength of the economy, the competence of the Emperors, the internal struggles for power, the religious changes of the period, and the efficiency of the civil administration. Increasing pressure from invading barbarians outside Roman culture also contributed greatly to the collapse."
So, setting aside a lot of fiddly-bit politics, there are a couple larger levers in there we might pull.
Perhaps the Romans work out that lead pipes are a phenomenally bad idea. As apocryphal as the impact of this may be, it works for our story - going into the 300s the Romans are in all ways just slightly better off. History otherwise goes ahead, but their economy, population, and general competence are higher, and fragmentation does not occur.
Perhaps Constantine I dies of a pox at an early age. He, and Constantius II, are never Emperor, and their abuses never happen. Christianity never becomes the official religion of the Empire. Juilan, not quite so burdened by the prior emperor's mistakes and abuses, is far more effective. He cleans up a lot of corrumption, and survives his Persian campaign, and does not cede any lands in the process. Having beaten both Germanic tries and Persians, the Empire is well-defended in the coming decades. Stilicho doesn't exist/happen. Our basis is then essentially a Julian Empire that has learned to control its taxation and while expansionist, is tolerant of many religions.
Either one of those works as a basis. Perhaps tomorrow, we can plow through a few other socioeconomic differences of this world that may give us some idea of what the future is like...
One possibility - Rome doesn't fall, because it doesn't become the Imperial Rome we know. Perhaps Julius never crosses the Rubicon. Or maybe, when he's assassinated, the optimates' view held - they killed Julius Caesar, Mark Antony, and several of the others, leading to a shaky but successful return to the republican form - we have the Roman Imperial Republic...
But nobody wants to model the Republic. They want to model the Empire with a Ceasar. So, let us consider the more common points of failure.
To quote a common text of Wikipedia, "The Roman Empire lost the strengths that had allowed it to exercise effective control over its Western provinces; modern historians posit factors including the effectiveness and numbers of the army, the health and numbers of the Roman population, the strength of the economy, the competence of the Emperors, the internal struggles for power, the religious changes of the period, and the efficiency of the civil administration. Increasing pressure from invading barbarians outside Roman culture also contributed greatly to the collapse."
So, setting aside a lot of fiddly-bit politics, there are a couple larger levers in there we might pull.
Perhaps the Romans work out that lead pipes are a phenomenally bad idea. As apocryphal as the impact of this may be, it works for our story - going into the 300s the Romans are in all ways just slightly better off. History otherwise goes ahead, but their economy, population, and general competence are higher, and fragmentation does not occur.
Perhaps Constantine I dies of a pox at an early age. He, and Constantius II, are never Emperor, and their abuses never happen. Christianity never becomes the official religion of the Empire. Juilan, not quite so burdened by the prior emperor's mistakes and abuses, is far more effective. He cleans up a lot of corrumption, and survives his Persian campaign, and does not cede any lands in the process. Having beaten both Germanic tries and Persians, the Empire is well-defended in the coming decades. Stilicho doesn't exist/happen. Our basis is then essentially a Julian Empire that has learned to control its taxation and while expansionist, is tolerant of many religions.
Either one of those works as a basis. Perhaps tomorrow, we can plow through a few other socioeconomic differences of this world that may give us some idea of what the future is like...