The Dreamer Lies Still

After mulling over my ideas for a new campaign setting with a friend I’ve come to some indefinite conclusions about how I want the game to play.
Firstly, the rules should either be B/X, a similar retroclone or a modified version of Into The Odd. Like ITO there’s a focus more on problem-solving and the use f items rather than magical hand-waving. I’m not sure if I’ll scrub rules for magic entirely since most of my players would be expecting something like 5e. If magic does exist then it comes with a great cost and a high risk, only the most nefarious and wicked would seek shortcuts to fate’s design. The characters too shouldn’t be too heroic, Elrics more than Conans, who rely on their forward planning to survive - it shouldn’t be uncommon for a player to have more than one character at a time.
Characters will also start with debt, along with a failed career (a la Electric Bastionland) and a list of contacts, some friends some foes. These are mainly there for more hooks, though I might revise this later if it shows to be too much choice.
Here’s where the general structure comes into play. Ideally it should be a west-marches like game, with player hopping in and out to suit their characters wants and needs. This means that in something like ITO with no inherent levelling system it would put the burden of motivation on the players to grow their characters. Additionally players should be moving around the map constantly, shaping the world as they go.
I’ve had some ideas about the setting but I don’t want to tie things up too much before I get a game or two in. It’s important to test the waters and see what kind of game people want to play in terms of narrative and function. On the broad it’s a grimdark, post middle ages setting in an emerging empire rife with uncertainty.
That’s all I’ve got for now, I’ll dive into Cairn 2e and a few other systems to see if I can find a good fit.