[u][i]"Could you clarify what you’re looking for, exactly? You say you’re in need of a GM but seem to already have the foundation of a setting and narrative premise laid out. Traditionally, that’d make you the GM - are you looking for a co-GM to share the workload?"[/i][/u]

I don't know that there's a real term for it, so I'm having difficulty putting the words down, and that's why the post is phrased the way it is. I could be wrong though, so if you have a term for what I'm trying to get it, do let me know: what I'm looking for isn't a traditional GM/player relationship/narrative, but a collaborative worldbuilding & exploration effort that is done primarily through roleplay. 

I guess more specifically, it would be a co-GM relationship with each of us playing a cast of characters, but centered primarily around the main character (or the villain in this case, if he qualifies) and a recurring cast for certain story arcs - but moreover, I'm also looking for character development and the exploration of certain moral quandaries (ie "do the ends justify the means") through the main character's growth as the roleplay progresses.

That's maybe the best way I can explain it - I had a better way to go about it in my head, but lost my train of thought somewhere along the lines. I have trouble putting my thoughts to words, sometimes - I might clarify a little more later if I find a more fitting way of describing it.

[u][i]"One thing to keep in mind is not to get stuck in the ‘worldbuiling phase.’ RPs die before they start because people stress minutiae and background details and keep adding more and more superficial elements without actually getting the RP started."[/i][/u]

That's just it, is the thing. The entire process IS a worldbuilding phase, but it's also meant to be a story. Rather than flesh out all of the super fine details outside the roleplay in OOC, I prefer to flesh them out through dialogues, descriptions and events within the roleplay itself just as the story progresses. It's not the most structured way to do things, but it leaves a lot of room to be able to adapt and build on ideas without it turning into a matter of "well before we start things we need to lay out all of the painstaking details about overseas trade routes and how this city acquires and distributes its food, and manages its economy, and-" I'm sure you get what I mean. 

Rather than focusing on all the little things all at once, it'll be a slow-burn introduction to them [i]as/if[/i] they become relevant, and whatever isn't deemed important enough for either party to bring up at all can just be left vague. It's worth noting that the main character in the case of this roleplay is a foreigner to where the majority of all this is taking place; so he's learning as he goes, and has an impetus to ask questions and figure things out as they become appropriate or important.

[u][i]"The most important thing in a RP should be the characters, as it is in traditional novels and literature. And players are responsible for those characters."[/i][/u]

Completely agreed. I guess it would be less accurate to say I need a DM and more accurate to say "I need a partner who wants to help me flesh out a world through roleplay, and is willing & able to play more than one character on a consistent basis, and probably a fair few of them". I don't know; I'm fine enough at the roleplay thing and a lot less great at the "explaining my thoughts OOC" thing.