I've DM'd once and I've only played a couple of game. I tried to get a few started online, but it's a lot more difficult than I imagined. My preferred is 3.5, but I could do pathfinder. I like 3.5 because that's what all my books are. :\
The biggest thing I'm doing is creating my own setting called Alabastra. It's fairly generalistic in a gamepaly sense because I don't want to mess with balance if I can help it. Also, I want people to be able to transition it to other editions.
It's called Alabastra
Right now it boasts a complete cosmology including the planes for all alignments, the planes of the gods (dubbed affiliated in the setting), the elemental planes, and the material plane one which the planet resides.
It's races include: Two types of dwarfs, two types of elves, humans, gnomes, halflings, half-orcs of two kinds, an extensively reworked merfolk player class, and a new race of winged bird people called the Buteo with half variations for both merfolk and buteo.
Each race has also been exstensively reworked setting-wise. For instance the Arboreal Elves (traditional elves) have a caste of tree-dwelling ironwood armor wearing samurai (renamed naturally).
It has just about every core class from every damn manual. Most are locked setting-wise but I offer up just about every variation in the book.
If it's from 3.5 it's in there most likely.
The entire planet is mapped into continents and regions. Several of the realms and kingdoms are in place, at least on paper if not geographically. I even have some of the bigger trade and merc companies.
It boasts at pantheon of about twenty plus gods and goddesses. Each one has a church that will be able to be joined and will have all its details, regalia, and ceremonies for reference.
It also has some of the basic rules of magic sketched out as far as the big stuff.
What the setting manual still needs is the prestige classes (I hope to create about half... but it's hard).
Also, there are at least a dozen weapons and armor that including things like harpoons and armlets that act like bucklers and allow the use of two handed weapons (but so do bucklers in reality), throwing knives, those sectioned staffs that monks use, and the meteor hammer (the monk weapon).
I need to comb through spells that I don't particularly care for (making a cabin out of nothing, never rubbed me right).
I also plan to write at least one hundred pages of history for the setting and one module (currently titled, "The Return of the Mad King"). As well as a monster manual of sorts. Though as always judgement will lie with the DMs of course.
My plan is to distribute it in PDF format for absolutely free, for anyone who wants to play or update it for a more recent editions.