I think we can safely say that the Roman Empire was eventually going to stop expanding, which means they were eventually going to run into a labor shortage leading to a fundamental shift in their economy. Thus the coloni, free sharecropper tenants, will still emerge as the new basis for agricultural labor in the Roman world.
With those broad outlines set, how does the West avoid the evolution toward what eventually, OTL, became medieval serfdom? The reforms of Diocletian and Constantine are generally considered a turning point in this decline, but I've also heard that the Crisis of the Third Century and concurrent hyperinflation didn't really leave them a lot of options in that respect. So what would be the best PoD?
A Principate that doesn't decline as quickly? Or perhaps, alternatively and ironically, it would have been better if Rome had not recovered from the Third Centuey Crisis, preventing the Dominate established by Diocletian?
If we can find a PoD that works, what would be the effects? What would the economy of Europe look like over the next few centuries, without the manor system propping it up? And what else can be said about how history is thus changed?
CONSOLIDATE: Just to get it out there -- does this lead to a Roman Capitalist Revolution, and possibly to a subsequent earlier Industrial Revolution?