Personally, I think I would look toward Northrend. Northrend isn't central to anything, so it wouldn't be the most convenient location ever, but I can't think of a cooler place than Dragonblight, Grizzly Hills, or the Howling Fjord to have a command post. They aren't my favorite zones in the game, Zagarmarsh claims that title, but they have exactly the right vibe for that kind of thing. A dangerous, imposing frontier? A possible location for future, civilized settlement by the Horde and the Alliance? It would give a sense of having a purpose in the overall scheme of things beyond the current war effort on the latest front, which is pretty cool. What about you?