A Friend of mine, who I shall call "Biffo" (Warduke knows who I am on about) 'pinched' my Game World and faffed around with it.
Guildstown became an Oligarchy - run by the heads of each Guild (I wonder what happened to Duke Osric whilst all that happened) and any real 'flavour' to the world started to leech away under his strict and slight unimaginative boot-heel - he 'pinched' almost ALL his plots from other sources, mashing them together as he went along, I kept trying to convince him to run pre-written scenarios (more as a learning experience for him than anything else) but he simply wouldn't. Whether he thought his ideas were brilliant and his games were great I don't know.
My personal problem was this, I was a DM who also enjoyed playing - so when the group Biffo was running for started to fall apart (the couple who used to host the game got divorced, I only got to play a couple of sessions with them - it was a shame, they were nice) and the game shifted to my good friend Chris's house (which was easier for me to travel too).
My previous group (which Biffo had played in) had started to fragment as well, one of the Players had taken the other Guys Girlfriend off him and yet another Player was at the start of a serious illness.
So I became the "Co-DM" for the Group at Chris's House.
Now my attitude to being a DM comes from Gary Gygax himself, you have to be "the Master of the Game" - never needed to refer to the Books whilst running a game, unless something unexpected happened (which to me is as it should be, your notes as a DM should be thorough enough) - one of the reasons I don't like random tables whilst running a game is because they can make you dependant on the books.
Biffo's was somewhat different. His word is law (fair enough IMHO) HOWEVER, that wasn't tempered with any sort of fairness or balance. If he didn't like someones character for any reason - he picked on them. Not good eh'.
But the worst came, when he took offence at the PC's getting around too easily and started the "Horse Plague" . . . . .
He had no thought as to what such a thing could do to a campaign world, MY Campaign World at that. Both myself and Biffo set the Games on Lemuria, and the implications for a Horse Plague on Aerth (and before anyone thinks to ask - I was using Aerth in 1976 some 16 years before it was used for Dangerous Journeys LoL).
No Horses = Less Trading = Increased Prices = Struggling Economy = Poverty
THEN you have the possiblity of War (you have Horses and we don't, you won't share - so we have to take etc, etc)
Something that never came into play when I was running games - but was always there in the back of my mind was the idea of Ferrets. Ferrets were meant to be (sort of) like the Fagins kids of Guildstown, young boys who WANTED to be part of the Thieves Guild but weren't old enough or lacked the Dexterity to join. If Biffo was bothered by PC's getting around to quickly, why not have their Horses stolen. Of course, the PC's would then have just bought more Horses - except, with the constant spate of Horse Thefts - they would not only be in short supply, but really REALLY expensive too (meaning the Party would have to adventure to get more money to buy the Horses).
That way, we have you more localized games - with lots of plot potential - c'mon Biffo - surely its not that hard to figure out!
How to end the Horse Plague - as the Greek Gods are worshipped on Lemuria (along with the Norse Gods, when Christianity took over on our world - the Gods migrated to Aerth - the two exist together in paralel universes), simply have Athena end the Horse plague and put things right. She was supposed to have shown man how to domesticate the Horse AND was supposed to be the inventor of the chariot - I can't see her ALLOWING a Horse Plague to continue!
I think all the above illustrate how hard it can be for two DM to co-run games within the same Milieu.
Chris (for example) sets his games on Aerth, but they are set on an Island off the Coast of Lemuria - Lemoy. Other than PC's sometime having to travel between Lemuria and Lemoy there is no real hassles!