Sounds like a clear idea for a nation RP.

I'm going to spit out some ideas on the spot: I've RP'ed before but nations RP are...more or less new-sort of had one or two..yeah, sort of. Sort of like you get hit by a car by getting your toes crushed.

The Atlantic Union: A political union that united after ww3 with the main focus to destroy the use of nuclear weapons-and weapons that may cause another MAD scenario- Made up of any tiny nations in its major aquatic base that tried to rise but failed. It has bad history with most nations due to offering rebels, dissidents and counter cultures and "defeated" factions homes.

Reformist Referendum: Has a series of research bases on Mercury(owns it in practice. Dosen't in theory) Was one of the major governments of mars but lost colonies due to Earth imports voting new ruling parties as a part of a secret service against the rising RR party. Lost capital in ww3 disolacting it from Earth. Shamelessly uses mass nuclear energy. Strongly transhumanist. Terrifying land forces and orbital power due to their strong roots in living in harsh planets and augmentation. Splintered territories. Not a nation(anymore) but a united cultural movement for forced regulated augmentation and defanging Earth political authority. they are a massive asteroid mining power due to their distance to their main manufactures and experienced space population. One of the real space factions due to their utilitarian and "anything goes" mentality to production and tech.

(I know, you said no space factions but I had to throw one out there. How could I resist)

Lost Liberators: Established by an off shoot of american survivors. Guilt ridden by their "Trigger Nuke", they wrote in their constitution that no nukes shall exist on their land. They have a base on the moon, an establish colony on Mars(originally nationalist american but switched ruling parties) however they mostly are located in Ethiopia due to leaving America. The forces fixed a collapsed system at the time. As a result they see themselves as the worlds therapist and doctor. Values economy and state involvement. Regularly annoys people with its AMSP- Anti-MAD Space Program- regularly placing orbiting satellites from mars/Earth designed to counter missiles. Very good at counter tech but unremarkable army in general. Understanding of orbital production is formidable.

I'll add more if these interest you at all.

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I'd say that the 1st one is my personal favorite, but all 3 have good and interesting things about them.

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What if I combine them into one idea then: Let me crack the whip here. It's a little late for me so excuse any mess ups.

Border Union: Has strong colony ties on mars but its main capital is an aquatic union on Earth. Was originally a privatized luxury town with new tech, however it passed powers as companies fell and ownership came upon a vote. Has a strangle hold on waters and gets into regular disputes over water territory- claiming a lot of it for it self. Powerful both to its mars colony for mining and its water colony for both naval trade/power and under water energy production that works 24 seven no matter the weather or conditions. The Union is split on Earth: One consider it over due for obsolete, desiring to destroy nationality so it can lead a humanity space age. The other side, considers important and so wants people off it to begin a true restoration of it. A small unit of power, but controls many weaker nations by strangling their naval trade. It uses people fear of a new war after ww3 to its advantage and has a ruthless secret service that regularly corrupts other nations voting process.

Despite is federal powers being back stabbing and ruthless, the people themselves are soft and typically carefree. They hold a lot of activist charities and aids to countries still recovering. They unwittingly bolster the image of their politicians that split from teary eye buffoons to cold and cruel one track minds. A lot of these folks were rich Americans and carry a guilt for the nuke war. Declaring war on MAD tactics by popular vote. It was constitutionalised by a unanimous vote among the people and the ministers.

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Interesting, I like it.

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How many are you hoping for in order to start this?

I'm game for trying a 1 on 1 nation if it comes down to it, you might have to tutor me a little. I've done nation lightly in games but not in forums.

Here are my ideas.

Reich of the German States: A state based in Germany, but expanded beyond the pre-war borders to include the Low Countries and parts of Denmark, France, and Austria. Founded by a warlord who wanted to be remembered for founding an empire, and who promptly did so by reviving the statehood of Germany and creating a political dynasty. It is a military dictatorship with descendants of the founder ruling the nation as Archon, below them being the nobility and their council, the House of Lords. Founded by an atheist, many of the secular reforms of the pre-war era were kept, and the state courts religious leaders only to appease their followers.

Republic of Lithuania: A state centered of Lithuania, containing not only Lithuania proper, but also the rest of the Baltics, Belarus, and parts of Finland and Russia. It is a Presidential Democratic Republic, but it is also a heavily nationalist and imperialist state. It holds sway over a number of proxy states through Eastern Europe.

Bengal Republic: A state based around the Bay of Bengal, with territory in the north-east India, Bangladesh, Bhutan, and Myanmar. Its capital in the West Bengal, its capital being Kolkata. It is a nation based and focused on trade, and its state religion is conservative Hinduism. Its government is a Republic which is ostensibly meritocratic and democratic, although this is fairly limited by its strict adherence to the Caste System.

The New American Republic: Officially, this state calls itself the Second United States of America, claiming to be the remnant to the since dissolved state already torn apart prior to the Third World War. Its territory is based in the Midwest, especially the Rust Belt, and has recently expanded into Pennsylvania and New York. It claims to be the natural successor to the state of Washington, Jefferson, and Madison, but it does not follow the Constitution of 1789 it claims to adhere to, being firmly authoritarian with a military dictatorship, state-controlled media, and national secret police, among other things, all in the name of a temporary state of emergency. It puts a large amount of focus into industry and research, and wishes to expand across what was once the United States and the Nationalists States of America.

The Papal States: The Papal States is a nation under the sovereign of the authority of the Pope. A theocracy, its politicians are bishops and cardinals who oversee a conservative nation that adheres to Traditionalist Catholicism. It aims to influence Catholics and Catholic nations around the world, and perhaps even beyond.

@AltRightHeroI had a similar idea before. I made interest thread here but it turned out to be more successful on a different site:
forums.sufficientvelocity.com/threads/..
Feel free to steal bits of inspiration from there if you want.
As for nation ideas I have a few but I am sort on time to explain them in detail.

- Kingdom of Neo Hungary: Basically a constitutional monarchy with the old pre-ww1 Hungarian territories.
- Socialist Republic of Germany: Think of a neo communist Germany which builds on the fruits of the 22th century science rather than Soviet ideas.
- Helghan Empire: A Greek-centric fascist empire.