The ability for a Junior Partner in a personal union to usurp the role of Senior Partner.

[IndigoRage]

This is based on historical precedent such as in the case of the Jagiellonian dynasty in the context of Poland and Lithuania and the Stewart dynasty's move to London from Scotland. Perhaps the player could literally swap to their Junior partner and have it become the Senior partner at will (this could be an interesting alternative outcome to what England does when it wins the Hundred Years' War) or the Junior partner could pressure the Senior partner diplomatically or militarily to become the dominant member of the union. Perhaps it shouldn't be easy to do so that intended independence wars like that of Sweden are still more likely to occur, but I still think it would be interesting.

Really, if it would be impossible to balance or even program it's not a loss, but I do think it would be a good addition for mission trees/some unique decisions for countries.

 

[Blk82]

The player is playing the "spirit of a nation" not a character. So, the player can't hop from Scotland to England, or from Navarre to France. There is a game-balance issue, in that in prevents OPMs from inheriting major tags "for free." For example, you could start as Denmark, switch the PU to Sweden, then avoid the Danish events. Or, you could start as Denmark, switch PU to Sweden, then release Denmark, winning your independence war for free and getting Norway with it for free.

There would also be the problem of tag-hopping to exploit aggressive expansion. If you can swap between tags, then you can dump your aggressive expansion on the tag you abandoned. As aggressive expansion is generally the limiting factor in expanding, this is would be a clear exploit.

However, the real problem is multi-player. For example, in the Denmark-Sweden situation, what if they are both player-controlled? The person playing Denmark could basically force the person who started with Sweden to actually play Denmark. Or worse, Denmark could switch to Norway, then to Sweden, to reduce the person who started as Sweden to play Norway as a subject of Sweden.

There is also the problem of hopping to PU subject, engaging in shenanigans (like deleting buildings, giving gifts, deleting troops), then hopping back to the original tag. Or, even without switching back, you can still take loans to near bankruptcy, gift to your PU subject, and get money essentially for free.

 

[IndigoRage]

The player is playing the "spirit of a nation" not a character. So, the player can't hop from Scotland to England, or from Navarre to France.

I mean can you honestly tell me anyone cares about a conceptual quibble like this? As long as you can culture shift and form a chain of formables this argument just holds no water to me.

I do agree with most of your mechanical issues (well except OPMs not being able to lead reasonable hold personal unions with larger tags. That's stupid anyway), but you're basically going with the most flexible version of the concept possible when you're coming up with them. And that's fair enough because I did at first say instant and not suggest a cost. If we go with the version of the idea where it's limited only to specific tags via unique decisions and missions I don't really see where the problem lies. For example, even if it's not actually a specific mechanic I feel like becoming France and England becoming a PU is a much more elegant solution to England winning the Hundred Years' War than the new "Angevin Empire" tag.