Mechanics necessary for a West Marches game?

Just kind of curious here.

If someone were to make a rules-light game system designed specifically cater to a fantasy "West Marches" style game (google "West Marches" if you have no idea what the hell I'm talking about), what would be required of the rules to best reflect the gameplay?

I know there would need to be tiered levels of play, and tiered monsters, so that there can be significant differentation between zones. There needs to be ways to facilitate travel over distances. Item differentation so that PCs will trade their loot. Differing character roles (but how much should they differ? Do you want a game where "every party needs a cleric?").

I'm just kind of looking at this as a thought exercise at the moment.

Well, I would say that most if not all of the type of mechanics present in D&D and Pathfinder (not the mechanics themselves, but what those mechanics are for) are necessary. Expanding on that, I think you need more mechanics to cleverly support exploration and travel. I don't know what that looks like per se, but I think that is the main area that current RPG's are lacking in that a GM for a West Marches game needs to figure out how to make engaging.

Oh damn I just realized this post is absolutely ancient. Sorry.