The Mighty Empires rules are buried at the core of my ongoing, but very slow, 6 player campaign (see
viewtopic.php?f=15&t=2889). They have been heavily modified over the years, both incorporating rules from other campaign systems, as well as me using my historical and military studies to tweak and alter, but the most important aspect to narrative campaigning is a GM.
The simplest way is simply adding a GM to straight Mighty Empires, letting the players see the map as in boardgame style play, with the GM adding story and scenario ideas to add colour. The middle ground is to have a GM with a secret map which the players never see, with the players plotting their own (and what they can of their enemies') progress on their own map, based on end of season reports the GM produces, then producing orders to give to the GM at the end of season phase.
Then there is the full on approach, which is definitely where I am at: A secret GM map covered in counters and markers (etc), as well as gameworld maps, with the players not even having full descriptions of more distant places, but instead having a version of the GM's map only fully detailed in their home and neighbouring regions, and maybe areas their PC or realm has strong connections with; full incorporation of roleplaying, in the sense that the players are actually one character (a ruler of some kind) and the rest of the world is NPCs, although each player commands the NPCS in their realm and army (though even that can be shaky in the full on format); full and vivid reports, sometimes ongoing during the turn, especially if a PC ruler is actually with a force or hands-on in some other way; strong NPCs; fully painted armies; illustrated stories with photos galore; long phone discussions, or (even better) pub discussions; constant adding to the rules, in response to whatever the players want to try to do, with the GM inventing, costing, recording the rules, then applying them (with lots of secret die rolls). The GM even plays the part of every player's advisers and military staff, in terms of telling him what they think if he asks, or what they suggest could be done. That way the player needn't feel stuck, but has several ideas to choose from (be they good, bad, indifferent - the player has to work that out). Interestingly, more than half the time players come up with an idea I haven't suggested, which is usually a better idea. That's when I kick myself and remind myself that I would not personally be machiavellian enough to survive as a ruler in the game world!
This very last element allows the GM not to feel like they are interfering, or steering the story, as not only does the GM no know what the players are going to try do to or order, but even the NPCs' actions are not wholly under their control. For all important decisions for NPCs, in response to situations brought about by both other NPCs or PCs, or in response to opportunities that arise, I write a little list of approximately 3-6 possible actions, then roll. I often include an option of 'Or something else' and if that comes up I have to create a new list. The first list is usually quite sensible, if varied, responses. If an NPC ends up rolling on the 'Or something else' table things can get a bit whacky. I would then try to remember and make that NPCs' next list a bit more unusual than normal. I do all of this without bending the rules or fudging, not due to religious conviction or a sense of 'honour' and fairness, but because the end result is that I too am watching a story unfold without actually knowing what is going to happen next.
I used to do all this without putting an illustrated account like a meandering, part-graphic, novel, onto the web. Now that extra element has become the massively time consuming bit that slows the whole exercise down to a snail's pace at times. Currently I have all the players' orders in, but have yet to collate them and print them on a big sheet and start to move markers around whilst e-mails fly back and forth. The reason I haven't moved that side of things on is that I want to do another photo story piece and haven't yet finished painting the 18 new figures I am going to use. Then I'll have to draft the story, create the scenes, take the photos, upload, photo edit, write the story and publish! Then I can go back to being a proper GM again!
Full on needs very patient players. It's exponentially slower than cricket in pace.
I could write here for days about this sort of campaigning, but I must to bed. If you wanna ask specifics please do. I could even paste aspects of the rules here.