I've always had a problem with making up my mind when it came time to choose which game I wanted to run. As a DM/GM, I've often had a serious case of attention deficit disorder (that is, ADD...not to be confused with AD&D). Lately, with my bourgeoning Midnight campaign, I seem to have hit a good stride. I don't feel the overwhelming urge to lose interest in the game in favor of another one. Sometimes the players and plot click just so.
That said, there are other games I'd love to run long-term. Warhammer Fantasy is one of the ones I'd really like to run (and play, come to think on it), but even I have certain limitations. I'd considered running a WFRP campaign in conjunction with the current Midnight game, but I'm starting to think it's not going to fly just yet.
I'd also love to try my hand at Eberron. While I wouldn't mind running the game, I'd probably prefer to play it. There's always less sacrifice involved with playing a game, as opposed to moderating one. I'd have to make time to really immerse myself into the setting (something I've done in relation to both WFRP and Midnight, due in part to the fact that I've been writing professionally for both settings), and I can't say that now would be a good time to do so.
Of course, there are classics, like Forgotten Realms. The current incarnation of FR is one of my favorite settings. I don't mind keeping current on FR releases, and I fully intend to start up a FR campaign when I have both the time and the inclination.
And then there's the setting of settings, at least in my book: Dark Sun. The setting hasn't seen a complete conversion to v3.5, though there have been attempts (both Dungeon and Dragon magazines had Dark Sun-related articles some time back, and there's a web site -- Athas.org -- that prides itself on the wealth of 3.5 Dark Sun information it contains).
This would be fine, but I'm a very "hands-on" type of DM, and that includes my preferences for Dark Sun. In other words, I don't want to use someone else's interpretation of my favorite AD&D campaign setting, so I'll convert it myself. I've already got a good deal of that task taken care of, which means that I can probably start up a 3rd edition Dark Sun game anytime I please.