One of my favorite RPG settings is Fading Suns. I routinely forget about it for a year or so here and there, but I keep coming back to it. One of the things I admire about it is the way the just about every story you can think of can be told with the setting. High fantasy world-saving stories, alien prophecies, zombie apocalypses, mythic quests, gritty and dirty politics... It's got it all.
But what does having that much room buy us? And alternately, what is the risk in breadth? And can we avoid some of those risks?
A game with a narrow focus is likely to be able to have better key mechanics. I slightly yearn to play Trail of Cthulhu for a good investigation game (with rules designed for investigation games!). This type of game, however, is probably no good for anything other than they're designed for though, whereas Fading Suns can handle a lot.
With the Rule of Law, one of the things that I envision is the Chinese Rome notion, where all roads lead to the Empire. The Empire is a melting pot of cultures, but is still threatened by barbaric hoards and the diabolical (maybe even literally devil-bound) Elven Kingdom. I like the freedom to tell and retell the great myths and stories of human history. I worry that the game/setting might lack focus, however. In a game with everything, what's to stop the players from rolling up a paladin, infernal warlock, shaman, and inquisitor? How do you fit characters from the Arabian, Chinese, Incan, and Iroquois cultures in the game together?
That's why I'm interested in building a small number of possible regions for a focus, and using alignment for the party.