I think we should be matching the mechanics a bit better. Absolute Crown Authority in CK2 does not actually equal the king owning and directly controlling the land, as complete incorporation implies. To quote the game directly:
The important thing, therefore, is transfering power from the vassals to the liege, while the vassals themselves remain nominally independent. Inheritance from characters outside the realm, and vassal opinion (relations) will be implicit, and therefore the only thing we need to do is, when converting armies, to move a great deal of the vassals military power to his liege. The effect is a weak vassal and a strong liege, as the CK2 mechanic implies.
With "Autonomous Vassals" such a transfer will not take place. It's tempting to make them independent, because they are "Autonomous", but this ignores the fact that they are also "Vassals".
However, there is the problem of "Minimum" and "Low" crown authority vassals being allowed to declare war on each other, which is not the case between EU3 vassals under any circumstances. Perhaps they should be in the lieges "sphere of influence" instead, which implies a much more loose "diplomatic" authority, allows vassals to declare war on each other, but gives the liege casus belli if they do.
So I would propose the following ruleset. The numbers are just taken from CK2 crown authority rules, so we can change them if they are too much or too little. I'm also assuming that we are converting military they way it was done in DV to HTTT, which was converting available levies in some way to their equivalent EU3 regiments.:
Autonomous Vassals: Sphere of Influence only
Low: Sphere of influence, 10% of military strength is transferred from vassal to liege.
Medium: Vassal, 20%
High: Vassal, 30%
Max: Vassal, 40%
Given that inheritance and relations will be implicit in relation and inheritance conversion, the result is:
1) Vassals are permitted to declare war on each other at the appropriate level of crown authority.
2) Ex-realm inheritance is permitted or forbidden at the appropriate level of crown authority.
3) (Relative) Vassal military strength is appropriate for the level of crown authority.
4) Vassal-liege relations are appropriate for the level of crown authority.
thoughts?