I love the idea of living campaigns in which a group shares the same setting and universe regardless of who is DMing. My friend group lacks the communication and organization necessary to pull this off. We've tried in the past and flunked; we have a lot of eager DMs who all wanted to do their own thing, so it was a "too many chefs in the kitchen" situation.

I want to hear your success stories!! Tell me about your collaborative worlds. I want to hear all about them: the in-universe lore, the real-life stories of how they came to be, and how you kept it up and running.

I've seen a success: The island nation of G'eth as done by the Oxventurers. For a one-shot, the DM and a player switched places (with their own character each). The DM who started the setting is very rules-lax while the player is more rules-aware. It was the player's first time as a DM and a great success in my opinion.

I think it worked for them because the player had been playing in the DM's world for a long time and became accustomed to the setting before putting his character in idle taking the reigns. (...but that's just, like, my opinion... *insert The Big Lebowski meme here*)

(I've also seen the opposite with another group which resulted in two instances of the same world, but that would be for a different thread.)

my persolel campaign  is set on a world  that is made up of islands that have own theme fire,water,earth etc we have a half orc druid who is a ex assasin me a crimanol golith an warforged barbarion and a rouge who i forgot the race of and an assmir cleric who looks like a anime charictor

Haven't run a game yet, but I recently started developing homebrew for a nation in southern Faerun in the Forgotten Realms setting since thats where most of my group's games take place. The nation is called Amn, its a newer power in the area having grown very wealthy from trade and that is pretty much all I've kept from what I found from older editions. 

The basic premise is Amn was until ~150-200ish years ago the most northern province of the Calimshite empire, which is this large multi-ethnic/racial empire that grew from Calimsham to around the Shining Sea and Chult (think the Ottoman Empire crossed with the Roman Empire). Being a relatively new nation, Amn has to deal with various other powers both mortal and extraplanar who attempting to gain influence over the state as well as border skirmishes with Calimsham and its vassal territories. This constant vigilence is taxing for Amn which necessitates the need for mercenary work (which is where my players will hopefully come in). 

Currently I'm trying to flesh out the major factions in Amn, but the idea was to create a setting thats new enough that my group enjoys exploring but that is familiar enough that I don't need to homebrew absolutely everything as I go. 

I too have never Dm'd my own game yet, but have been working on a Homebrew world that takes bits and pieces from different editions and setting, with 3.5 and 4e being the biggest sources of inspiration as far as lore and creatures and what not. 

The plane's placeholder name for now is Salvera, and it is a high magic setting. Currently I have a continent of dragons, a massive span of land inhabited by "natural" mechanical creatures, a forest that is always experiencing nighttime inhabited by dark but benign creatures, and a massive capital city where the main castle doubles as a throne that seats a colossal warforge-like construct that has remained dormant for eons. It's been fun to brainstorm.