MNPundit, that's quite a bunch of good ideas. I didn't even think of monetary aspects.
As for Charlemagne, I have means (and plans) of
keeping him out.
More generally, I'd like to avoid a Francs-specifics, from which feudalism seems to borrow massively.
My greatest concern was the history of ideas, i.e. which systems could have been designed, understood, and accepted.
And this is always a really hard question ...
As to the possible choices, there is always the old continental Germanic system as with the Saxons and Bavarians:
The soil belongs to the free farmers, and the duke may lead them into war.
Is there any particular reason why this simple system cannot remain longer?
It probably never was that simple in the first place, that's the main problem. I'm not going tpo pretend I understand the Early Middle ages, and I've never met anyone who diud,m but I have a few suspicions.
- the first is the different nature of war. In an increasingly consolidated and organised state, war becomes a less certain, more arduous, less frequernt but more demanding undertaking. As these larger states become the targetrather than source of predatory raids, war also stops paying dividends. Many people who might happily have followed Readwald or Chlodovech into battle would have been unable to muster the resources and capabilities to stand in Charlemagne's or Desiderius' army. They could be legallyx forced to stand or be compelled, as a group, to provide a number of troops, but the military usefulness declined while the cost of enforcement rose.
- the next is the tax base. No Western European state after Rome had anything akin to a stable tax base. Often, giovernment depended on service obligations much more than revenues, and taxation was often contentious and foprcefully resisted. The Anglo-Saxon kings managed to build up a sort-of viable system of royal land rents that probably had a future, but not many states managed to. 'Feudalism' or rather, a system in which control of the land meant control of social elite functions has as its main advantage that it functions largely without taxation ('Feudalism' is a later legal construct to explain what happened when nobody knew what feudalism wasd, and it doesn't describe the reality too well). Of course you could also find different solutions. For example, if instead of the Freaniksh model of widely dispersed landholdings you had locally contained aristocratic estateslike in the Lombard kingdom, you could develop urban or proto-urban communities as foci of royal control. Even untaxesd, they could easily develop into a non-feudal system of fragmented government, and they would also lend themselves to taxation. Military obligation woiuld natzurally devolve on all inhabnitanmts, with certain contingetrs for larger-scale operatzions. Something like that was tried, too, but the largely rural society of England and the Eastern Frankish Empire probably just couldn't sustain it.
- then there's the question of land tenure. In reality it made almost no difference, but the idea that you could 'hold' land 'from' someonme and still be an aristocrat is an important part of the social developments in the feudalisation of Europe (can you mentally add quote marks? I'm very unhappy with the term). In Frankiish tradition, that was something that marked your out as a social inferior. Significant people *owned* land, dependents (and Romans) rented it. The net result would probably be a society in which royal estates would be either alienated completely, the Merovingian way, or kept as a source of revenues that led to a monetised retinue maintenance system. The king would pay his retaioners, as would major aristocrats, and land would increasingly become the 'it' resource, changing hands rarely and only for serious money amounts. At least until someone manages to secure alternative revenue streams.
- a final possibility would be a different turn to the development of unfreedom/dependence. Most Germanic kingdoms had significant unfree populations whose exact legal status is bloody hard to pin down. Under a feudal mindset, the broad category of 'serfdom' came into being. If Roman law obtained, we could see either the emergence of a more strongly monetised 'tenanbcy' or a straightforward reduction of the dependents into slavery. The latter would be especially dystopic, but certainly possible as the powers-that-be (landowning classes, from the small liber homo and ceorl to the bondi of Scandinavia and the greates maiores and magnates) applied the cruel letter of the law to a class of people previously treated as - by and large - junior family members.