I'm afraid that I can't do much for you here, I've neither played M&B nor run an open world campaign. Sounds difficult, honestly, but if you have a vision and can stay on target, anything's possible.
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Yeah even looking at it. I think I mightve bit off a lot here. But I've had the idea of a medieval fantasy mecha show since I watched Escaflowne a few years ago, and watching Dunbine right now has only invigorated it.
So I suppose I'll ask for two
1. I've played this system quite a bit, but it's always been balanced encounters. Can unbalanced encounters work? A big part of mount and blade on the map you'll see 5 Looters, while simultaneously there might be 78 Sea Raiders. 'd . So then it's time to stay clear of the. Obviously for everyone's sanity I would never even consider those numbers. But it'd be like. It'd be like if I put a PL5 boss on the world map and they players had to adjust their plans around it. If this is a fundamentally bad idea than it's better to learn that now.
2. Not a MnB idea. But I've been toying with this idea for awhile. But how do you feel about promoting Grunts into Rivals if they prove themselves? I've been pondering about this after a session that happened a year ago.
1) I mean, it can? Live Another Day exists for a reason. If it doesn't work, they can skedaddle. Kinda like when you try fighting an open world boss and it doesn't pan out.
2) Sure, ascending nobody NPCs to named character status with a story purpose has been done in fiction both interactive and not. A Grunt survives, paints their mech, declares their name during the next meeting, etc.