I Have An Idea - Kingdom Simulator

This is what you'd call a "Looking for GM and Players" or a "Looking for Everyone."

Basically, I have this idea for what I'd like to play, and I think other people might like it too, but I'm not seeing recruiting threads for it. Or I'm just blind and haven't spotted the obvious. If you would be willing to run one of these, and know which system you'd like to use, I'm here for it. I haven't played a lot, mostly just Pathfinder, D&D, and freeform, but I'm very willing to learn. If you like what's here but you want to play, please do drop a comment saying what you've got an interest in; just bear in mind I'm looking for any GM interested, so I'm signing up to learn whatever system they propose, and you should be ready for that too.

What am I looking for? Well, I've briefly played a little Kingdom sim in Pathfinder (it got cancelled very quickly), and tried my damnedest to play Crusader Kings (before discovering that I'm chronically allergic to that style of gameplay). Despite these experiences, I very desperately want to build up my kingdom from scraps, embellish it with developments, organize and mobilize armies, all the delicious bits of detail. Political intrigue is good, but the game must have a tactile feel. I should feel like I can accomplish something more than White Peaces, Casus Bellis, and sending a very impersonal bribe. I want to explore! I want to have tense diplomatic encounters. I want to fight over resources and artifacts of the past. I want strange characters showing up at my doorstep.

Please tell me someone here has a good system for that, and would be willing to run a game. You will have a very dedicated player if so.

Kingdom management falls into the wheelhouse of a game like A Song of Ice and Fire, or it's less Game-of-Throney offshoot, Sword Chronicles. However, any system could be used with a little effort.

Do you see this as a game with a single leading character, the monarch, or a group effort, and if the latter, how might that be managed?

There is a system called Kingdom which might be fitting for your vision. :D Although it's more about a group effort shared between players as opposed to each player having their own kingdom.
I think there was a game of the latter variety on this site once upon a time... So interest probably does exist.

Hey, if you go with a cooperative game, I'd actually be interested in joining as a spymaster of sorts. As long as you promise to not fire me from the council because a powerful vassal with 0 stats wants a seat. XD

I was thinking Conquer the World a la Rise of Nations or Risk, wherein different players have different countries under their command. But if the majority wanted to co-op a single nation, I'd be about that. I don't know what roles would be available to me in a co-op game, but FlyingSucculent apparently wants to be spymaster. I would like to be a part of trade or diplomacy.
See above. If we do co-op, I'm all about letting you be Spymaster.

There are lots of games that explore this space, and I wonder if @Phophos knows of any indie RPGs that focus specifically on stronghold and kingdom building...

Forbidden Lands has a whole strongholds thing it gets into, though that's not quite what you're after, Walker, but isn't there also both a Matt Colville book on this *and* a Pathfinder Adventure Path that is all about exploration and kingdom building?

I'll take a peek look at The Rebel Crown as well, as it might fit some of the political bill... (as might The Sword, the Crown, and the Unspeakable Power.)

Pathfinder's Kingmaker adventure path has some robust kingdom building elements (and is likely what you refered to starting briefly in your OP) but in PbP it would take a solid 6ish months minimum before you evem get into that aspect of the game.

I have ran it, however, and could recommend it. Its a fun adventure overall amd allows a lot of exploration. The kingdom building mechanics allow you to build multiple settlements and choose what buildings to construct in each, etc.

I have also played A Song of Ice and Fire RPG and I agree it is designed to fit what you describe fairly well. However there are not many published adventures for it, so someone would need to be familiar and comfortable enough to homebrew something.

Dirigible just pitched for a Sword Chronicle game, scroll down and have a look!

You should definitely check out Knight Guild. This is a new version of an all time classic Pbm that's coming out of beta and about to launch. There are 100+ players to date in a massive world that I think will tick all of your boxes. Game is combination of computer and human moderation. I messaged one of the game's seniors (an incredibly helpful guy) and I can pass you his discord and/or email if you'd like to know more.