When we get deeper, what I'd like to do for the story is have the DM and other member of the group to get together and write a story campaign and rotate through them, so each member... this way we can give ppl a feel of DMing by being a Guest DM (they could play the role of the villian of the campaign or something)
Another important thing: Do we want a long campaign or short modules?
Long campaigns have the benefit making people more invested in their characters, but modules lend themselves better to the Guest DM'ing you mentioned and drop in, drop out play.