This is a good question. I myself am planning to make my MC eventually run/have a heavy influence on a certain country, with him being one of the strongest people in the world.
My plan is just to start from said kingdom and make it the best possible kingdom on the face of the world. It would be the most technologically, economically, and culturally advanced, to the point that it can influence other kingdoms. Through that influence, MC could ban stuff like slavery. Kinda like how the British did it.
It would be the very first fully blown industrial nation, so it has no need for slaves, especially since I'm also planning of giving the technology of making automatas and golems freely to the public. It would also hold the knowledge of teleportation gates, massively boosting the ease of trade.
Through it, the world itself would be modernized.
While I'm not sure how you might change the views of the masses to not condone slavery in a preexisting nation, working from the ground up might work. Like starting your own nation/kingdom that has laws against slavery.
Edit: or like
@ForestDweller said just influence a preexisting kingdom from the top down.
Okay. From the two of you I'm getting the idea to start a nation with the first citizens being slaves. The way I would go about it is as follows.
Firstly, I would find a suitable geographic location to create a nation. I would spend a large amount of time/effort researching geography and the political systems and situations of various countries to determine the optimal location. I would anticipate that eventually I would come into direct conflict with countries over resources and territory. The location needs to be one that has good land, plentiful natural resources, and countries that I can handle one way or another.
To start the nation I would need citizens. I think it would be best if a nation that focuses on abolishing injustices like slavery was built by slaves. To procure my citizens I would travel to another locale far away, possibly on the other side of the planet, and teleport poorly treated slaves, secretly at night, to my new nation. If I was OP I would have a teleportation power right? The reason I would do it this way at the start is because I want to minimize all conflict at the starting stage. If I teleport slaves so far away from a country that they could never even find them, let alone that they would not be able to travel across the world to get them back, then I win. If I'm so OP then I could teleport thousands of slaves over a few years.
90% of the work will likely be governance. The slaves might have language barriers, low education levels, they might be racist towards each other, diverse religions, and many of them might not want to be part of my self-righteous social experiment. A rocky start.
Endgame would be a meritocratic nation, with separation of powers in government, a more or less flat income tax rate, an enforced maximum wealth gap (gotta stop the development of a plutocracy, oligarchy, and or new caste system based on wealth), militarized nation with mandatory service, universal suffrage, universal basic income, universal healthcare, universal education, a constitution, declaration of rights, state banks carefully controlled by constitutional rules, labour laws, child support welfare (Need more citizens. Be the opposite of China's one-child policy. Gimme babies. Gimme impressionable children to run through a well-crafted mandatory education ...), etc.
The reason I would have mandatory military service is because it's a dangerous world with monsters and constant war. Citizens need to be able to defend themselves. I would say that everyone would need to do a minimum of 1 year of paid mandatory service with the possibility of extension if they desire. It wouldn't be particularly dangerous service; it would be more like an extended self-defense class and apprenticeship. It should also have a vocational and academic apprenticeship learning component where people can choose their preferred training regime, like they also gain other trade skills and academic skills in addition to military training.
All the other things I mentioned also have similar justifications.
I would try to prepare the nation to operate without me. It would be naive of me to think that I would be able to support, govern, and protect the nation forever. If reincarnation happened suddenly, then it could just as suddenly vanish one day. What would they do if I disappeared?
I would probably avoid creating a theocracy. I wouldn't want to play the role of a god very much either. Maybe the real god of the isekai would get angry at me. XD. However, I had the nifty idea that I might want to make a fake enemy for the people to band together against. Maybe I would I tell everyone that I am trying to prepare the world to fight against the inevitable resurrection of the Great Demon God Diablo. *snorts in laughter*
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There is also the issue and benefit that a truly OP character is a walking nuclear weapon. For the most part, the beneficial part is deterrence. Rational people would naturally avoid war at all costs if the opposing side was overwhelmingly powerful. If wielded well it could lead to bloodless battles.
I opted to build a nation from the bottom-up to give myself more control over it's policies. However, I think that influencing other nations using the top-down approach is also a very good idea. Depending on the country, my methods might be insidious. The weakpoint of caste systems, monarchies, and dictatorships are the ruling class, monarchs, and dictators. I would probably carefully threaten the ruling class from the shadows to achieve my goals. Change these policies or I will kill you. I prefer this method over open war because it would greatly reduce the number of citizen deaths. If a dictator wants to start a war that would result in massive casualties I would rather just kill them immediately, and then threaten the second in command right after. It could be an effective method to prevent war ...
The problems with a bottom-up strategy where a nation is created is the low population relative to surrounding nations, and small territory which might only be expanded through annexation.
Step one: Do your best to minimize knowledge of your existence. Kill your way to secrecy if you really have to. The less people know of you, the fewer countermeasures they can put in place to stop you. Paranoid? Perhaps. But in a world that may have any number of magical abilities, paranoia is necessary in order to deal with the unexpected. Murphy's law is a real bitch. Plus, you don't need to love and adoration of the people, you need to fix the world.
Step Two: Investigate the powers and abilities of the world. You need to know every skill or talent different groups could have. Be paranoid. It's not paranoia if they really are out to get you. If there are any techniques or abilities that are straight up evil, do your best to remove them from the world.
Step Three: Investigate the world powers/countries/organizations. Understand the toes you're going to step on if you continue down this path. Determine their motivations, whether or not they would be more likely than not to be against you, and be friendly with them until you decide that they're not the people you want to deal with.
Step Four: Build up a power base. One man cannot do everything, even if they have ridiculous power. If you want to leave the world in such a state that it can function without, then this is necessary. Second opinions are also a major plus. Even if you can come up with good ideas, that doesn't mean others can't come up with even better ones. Basically, don't do everything yourself because that's stupid.
The other thing you need are assets. Money. Resources. The more you have that you can throw at problems, the better.
Another important thing to keep in mind is that people are people. The people you give power to do not have to be entirely dedicated to your goals, but they do have to obey your and be loyal. Letting your power base get full of spies is stupid and shouldn't be done. You should be spying on everyone else instead!
I like your caution. People who get drunk on their power and make themselves easy targets can be toppled by forces they didn't expect.
A good 'power base' as you put it is really important. Thank you for stating it well.
I completely agree with the need for comprehensive knowledge before any major actions are taken. It's impossible to make good decisions without understanding the current state of the world.
It's a handicap to be forced to only take actions from the shadows. There are times where I would want to directly take action. I think I could find a compromise between my desire to act openly and my need to remain hidden as a precaution. I would have some of my power base act as my proxies in the light, while I and the rest of my power base act in secrecy from the shadows.
Conquer the world by force and then enforce the eradication of injustice.
justice without power and force is just naive idealism.
I might be able to conquer the world but not their hearts. The enmity that I gained through violent conquest will likely persist far into the future and be a constant source of insurgence. At this point I would officially be a tyrant. Tyrants have to rely on oppressing the citizenry to maintain order.
Also, I somehow don't have the confidence that I would be able to conquer the whole world so simply. I have the feeling that many things would go wrong and that I would overextend myself.