Mercurius
Legend
Yeah, Crothian is right. The number-one thing you need is a good game.
The number-two thing you need is a good game that has a point beyond being, frankly, a more traditional version of D&D, because ain't nothing stopping you from running a traditional fantasy game with D&D.
Yes, I agree with Crothian. But let me clarify about "a more traditional version of D&D." My view is that because 4E is less traditional than previous editions, there is a relatively large group of long-timers who might be open to playing something else, something "more traditional." So the game I envision would have to accomodate that, but also add something new, which I am saying could be embodied through these theme/setting sourcebooks.
Does anyone remember the game Shards of the Stone from the mid-90s? It was similar to what I have in mind, with a Core rulebook (which is what it was literally called) and numerous Realms in the same world that were cut off by some kind of magical mist, most of them separate continents. Each "Realm book" (which never came out as the game collapsed under its own ambition and some in-fighting, if I heard correctly) would detail a different continent with a different theme or style, almost like separate campaign worlds. I think they even had a couple Big Names like Ed Greenwood on to write a Realm.
Now I only bring Shards up because I think the basic idea might work for what I'm talking about: core rules, one larger world, dozens of smaller realms/themes. Shards had a kind of metaplot that might not lend itself to huge popularity, but might work (see World of Darkness). If I remember, at least one of the Realms was pretty traditional, but some of them were pretty out there.
By the way, Sean Patrick Fannon's "Shaintar" PDF game is one of the (more traditional) Realms, Fannon being one of two main designers for Shards.