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?
I´ve commented on this idea (and even wrote a timeline) fairly frequently, since it´s a pet idea of mine. Not in the linked thread, though, but there were others in the recent past, too. (I remember getting derailed in a flaming row with LSCatilina in one of them.)
The Crisis of the Third Century didn`t leave Diocletian et al. with no other options; in fact, it opened up many windows of opportunity because it shook up established political structures, so many different outcomes are possible, and maybe even happening at the same time, in different parts of the Empire, if it remains splintered.
This shouldn`t be imagined as a total collapse, like
@Mikestone8 proposed, because then the feudalisation is inevitable. Rather, it could be a bunch of Empires trying different paths, and since they`ll struggle against each other, maybe the more efficient system comes out on top and imposes its structures on the rest.
Basically, here is my view of the situation in the 3rd century CE and of what could have happened:
There wasn`t necessarily an overall shortage of labour in rural areas. There HAD been over-urbanisation processes going on into the 2nd century, but by the 3rd century, this trend had begun to reverse itself, as trade volumes plummeted, specialised crafts lost their outlet markets, towns became preferential targets of barbarian raids etc. So, the large landowners actually had, on average, lots of people competing for wage labour jobs on their combined agricultural-manufactorial complexes. (If that hadn`t been the case, then Diocletian`s reforms wouldn`t have been applicable at all: coloni in a good bargaining position would have put up enough resistance for the feeble imperial administration to be unable to coerce them. As it was, though, people fleeing to those manors where there were employment opportunities put up only very limited resistance to having their rights strippped of them.)
Why did the change in the status of the coloni happen, then? Because things looked different in different places. By the 3rd century, the Empire was going through a serious home-made ecological crisis, especially because of massive deforestation, which caused erosion, which in turn caused a degradation of mountain soils (good soil washed away) as well as a degradation of river delta soil (swamping) because of silting. (It caused, at the same time, many port towns to fall dry, which damaged long-distance trade even more.) A mobile wage-earning rural workforce, which had emerged from the late 1st century CE onwards, was leaving marginal lards and flocking to where they could make a better living. They had no incentive to put in the extra effort to improve and restore the value of the lands (because they weren`t theirs), while those who had - the landowners - often were short of cash after years and decades of bad harvests and couldn`t afford to both pay their workers well AND invest.
To the imperial administration, which wanted taxes, this was desastrous. (Also, the landowning class was still, well, let´s say well-connected.)
In part, this is what drove Diocletian et al. to redefine the status of whole groups of society: not just the coloni, in theory. Urban craftsmen were supposed to be tied to their fathers` professions, too, but that one never worked out, because in contrast to the rural reform, you had no-one to profit from the measure and enforce it.
If you don`t want to go all utopian like my Res Novae Romanae timeline, which certainly isn`t the apex of plausibility, I must admit, then what you should go for is a different strategy for land improvement and a consodliation of the taxbase. This is the easiest to achieve really, easier than a revival of long-distance trade (which would have required improved security, which in turn required military might which we can`t just handwave into existence).
To do this in a way which wouldn`t reduce the coloni into serfs, you would need people in power who have the balls to confront the landowning elites. The only ones who could do that are charistmatic emperors / splinter-empire caesars who enjoy the full support of their troops, and who use just these troops to make their reforms happen on the ground. You`d have to take away fallow and degraded land from those owners who can`t improve it, and have soldiers improve it (desilting and drying out swamps is easier here than reforesting mountain slopes because the Romans maybe didn`t understand the ecological relations and reasons why that was important) and then own it, in the good old land-for-veterans-way. This wouldn`t necessarily bring more taxes immediately, but it would reduce the need for taxes in the middle and long run. Reverting to farmer-soldiers to some extent relieves the military budget.)
Or, to be much more general in my response:
It is not difficult to avoid the enserfment of the coloni, but what it would take would be military rulers who imply their own people (i.e. the soldiers) in rural economy instead of attempting to regulate rural economy through laws.