Since subject types aren't moddable at all, I might as well suggest some new subject types that would fit well into the vanilla game. Although if subject type modding could be enabled, that'd be great too. There are a few subject types that I think would work in the vanilla game, namely principalities, de jure vassals, and a new type of personal union.
Principalities
Principalities would be somewhat similar to a personal union, except that your heir rules it. If you have no heir then you simply rule it a bit like a PU until you get an heir. They, of course, won't be able to enter any royal marriages with anyone. Rather than liberty desire to be independent they should instead be able to try and rebel to try and become the king instead of their father. In the game Wales and the Dauphine would start as principalities, as well as a few others possibly. This is probably the hardest one to be implemented though.
De Jure Vassals
This type of vassal is one that is technically a vassal, but other than that they're effectively independent. It's not a massive change to the game, since the biggest example is Burgundy and they are independent, but rather than trying to conquer their land you will get missions and cassu bellis to turn them into a normal vassal again. Normal vassals that grow powerful can slide towards being de facto independent.
New Type of PU
I haven't thought of a good name for this, but this new type would represent a union where two crowns are effectively tied together and won't be lost upon the death of a ruler with less than 0 prestige or an interregnum and if you become a republic you inherit them. This could be used for England and Ireland - the King of England was also the Lord of Ireland (and later the King of Ireland) but there was no chance of it being lost. You would have more control over your lesser partner than in a minor PU and there is very little chance of them rebelling. Normal PUs can only have this happen if the senior partner has a higher government rank and the PU has been stable for a long time.
Principalities
Principalities would be somewhat similar to a personal union, except that your heir rules it. If you have no heir then you simply rule it a bit like a PU until you get an heir. They, of course, won't be able to enter any royal marriages with anyone. Rather than liberty desire to be independent they should instead be able to try and rebel to try and become the king instead of their father. In the game Wales and the Dauphine would start as principalities, as well as a few others possibly. This is probably the hardest one to be implemented though.
De Jure Vassals
This type of vassal is one that is technically a vassal, but other than that they're effectively independent. It's not a massive change to the game, since the biggest example is Burgundy and they are independent, but rather than trying to conquer their land you will get missions and cassu bellis to turn them into a normal vassal again. Normal vassals that grow powerful can slide towards being de facto independent.
New Type of PU
I haven't thought of a good name for this, but this new type would represent a union where two crowns are effectively tied together and won't be lost upon the death of a ruler with less than 0 prestige or an interregnum and if you become a republic you inherit them. This could be used for England and Ireland - the King of England was also the Lord of Ireland (and later the King of Ireland) but there was no chance of it being lost. You would have more control over your lesser partner than in a minor PU and there is very little chance of them rebelling. Normal PUs can only have this happen if the senior partner has a higher government rank and the PU has been stable for a long time.
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