Very much this. I can not understand why so many people keep saying things like "every nation should be able to use mechanic X" about every single mechanic. Diversity is the core of replayability. Dealing with the same problms in different ways. Facing different problems. Having stronger bonuses for one area of the game or the other. If every nation would be able to use the EoC mechanic, then what would be the point of playing in China. If every religion had access to the Papacy (just renamed), then what would be the point of having multiple religions.
Diversity and mutually exclusive options are what motivates one to play again and again. I remember Stellaris on release. It was horrifyingly bland. One playthrough was enough to see 95% of the game's content. What would be the point of playing again? To perhaps see the remaining 5%, but at the cost of repeating so much? EU5 needs to have diversity embedded in it from day 1.
I agree with the argument that diversity is required, i disagree with the argument that diversity should be achieved by limiting certain mechanics to certain tags or regions.
The choice shouldnt be: Will i play Prussia for a militaristic approach or Austria for a diplomatic approach?
The choice should be:
I'm playing the Netherlands because i want to play as the Netherlands.
Now will i conduct my nation to be a staunch Catholic and uphold the power of Rome or will I disrupt it by going Protestant?
Will I try to sieze power in the Holy Roman Empire or try to disrupt it?
Will i devote myself to be an economic powerhouse and dominate trade, or a military powerhouse and conquer Western Europe?
Will I attempt to form a Colonial Empire or restrain myself to my core homeland? And if I colonise, will i try to establish a large colony in the New World or go for a trade network in Asia?
Will i hold absolute power as a divinely ordinated monarch and centralise my country to ruthless efficiency or embrace liberal philosophical values as a republic or parlimentary monarchy?
Will i legitimise my power by virtue of a strongly upheld religious values and identity? Or by sustaining social cohesion by virtue of a culturaly homogenous nation with a strong ethnic identity? Or perhaps due to a certain set of philosophical principles and values that holds the nation together?
Diversity happens when all these options are equally viable if fully comitted to and offer completely distinct experiences when followed, while also being incompatible to follow all these options efficiently in the same playthrough.
I'm not saying all options should be equally easy to achieve, a country like the Ottoman Empire is in a much better starting position to follow a playthrough of agressive military expansion but also of cultural and religious tolerance. A country like Korea is in a much better position to play a more defensive mindset and focusing on domestic issues, and optimizing their homeland to maximum efficiency.
A country such as the Mughals would have a much easier time pursuing a policy of religious and ethnic tolerance in the diverse Indian subcontinent than to try to convert everyone to their culture and religion.
A country like Portugal is in a much better position to pursue a policy of overseas expansion and defense of their core homeland than to attempt to wage wars of conquest in Western Europe which is going to be dominated by much larger and well established realms such as Spain, France, Britain and the HRE.
In conclusion, diversity should be achieved by allowing every nation to take different approaches when dealing with the same problems, rather than designating certain tags to certain playstyles.