There is a very cool exchange happening here; I hope more players come across this post. Now to give my two cents on the post in general, I don't believe it to be fair for a nation to occupy one niche and for it not to be available for "an alternative" to be created. Yes, if multiples of the same niche are created, then the niche is overused and boring and leads to a dispersed player base.
However, "Inheriting nations" or rising through its ranks can and oftentimes are unrealistic.
If I had a Cossack Raev group that wanted to delve into "low-fantasy Slavic medieval RP" as you described Haense, we couldn't do so unless the group molded to the RP Haense has. You say nations should be inherited, and that players should change nations from within if they enjoy said nation's niche instead of going off re-creating a similar niche and diluting the player base.
I believe this is unrealistic. Cliques and Redtape are in the way.
Cliques are a part of human relationships, afaik. But to change your/a nation, you really have to be the NL or be friends with the NL, considering a PRO is the God-Emperor within his tiles. And to get to these leadership cliques for a nation to get new ideas can take an IRL year if it ever happens at all; why wait so long when you can spawn your Slavic nation next to Haense.
Red tape, rules, and whatnot, again PROship. You can't easily coup a nation or force the king's hand through threats of force. To have a revolution (which I think was a great warclaim addition, thank you mods), you must have a tile within a nation, so if you have any PvP-centered players within your Cossack group, why would the king risk giving you any land?
At the end of the day, people don't like their nations changed, even if it's completely good faith RP. You have a legitimate protests/riots against the current king? He doesn't fear for his life; he can log out.
You storm the castle and kill royals your characters deem corrupt? They don't want to PK.
You get elected into the Duma and wish to pass land reforms with the people's consent? Doesn't matter if every persona beside the king agrees with you.
The king won't give in. He'll evict you if you keep stirring trouble.
There are no consequences that players HAVE to admit to outside of the warclaim rules. Not even good faith change to your nation.
And who would even define if a niche is taken?
An exact copy of Balian could pull 100 things out of their ass and claim it makes them different.
Or a nation with a different niche than Balian could be denied their nationhood on the grounds its "close enough"
I know I'm not offering a solution to these issues, and this whole thing was a disorganized rant. I simply believe the status quo works better than having a restraint on niches.
[!] Wants to LARP Raev RP:
[Low-Fantasy Slavic Medieval RP taken. 1/1 try elves!])