Fair warning, if you try to detail the entire world before beginning the game, you'll never start. I'm a career game designer and I've run Dungeons and Dragons games for over a decade, and if you try to build an entire world in-depth then not only will it take you forever but most of the content will be wasted as the players only see bits of it and simply have no reason to go to some places.

The best way to build a sprawling, epic fantasy world is to have a general idea of the rest of the world, and highly develop the area where the campaign will start and perhaps some of the surrounding areas that you're interested in using as plot hooks. As the players move outwards, you can develop the details of the world in the direction they're going. Along with resulting in vastly less work for you, it also means that you have a lot more freedom since nothing is set in stone, and you can have things happen as you need them. For example, perhaps it would really help the storyline if the party stumbled into some small village after barely surviving a bandit attack- if you don't have the world detailed, you can plop a village down no problem. If you've already made all the details, then the party might just be shit out of luck, and could end up just dying in the wilderness as a rather anticlimactic game ending.

I like big fantasy worlds, but I also like the joy of discovery. It's easier for the person running the game and often more fun for the players if everything isn't set in stone from square one.

Anyway, I am interested. I've got an idea for a big, hulking, Lizardfolk voodoo witchdoctor that I think would be pretty fun. Fair warning though, he's not exactly good aligned.