Icons campaign going that I love (although I still haven't written up the most recent session and the holidays have, as always, slowed it down), so the thing I pine for is D&D. Despite the picture, it would be 5th edition. I just really like that old Erol Otus cover. Anyway, these are the main ideas that keep surfacing.
World of Greyhawk
I had this one going for a while, and had to drop it because I was gaming all the time and had to cut a few things. It started off with a weird vegepygmy adventure which led to Sinister Secret of Saltmarsh and then off into the Hool Marshes where they stumbled into the Feywild and then Castle Amber. I could easily pick this up again. While the time-sink was unfortunate, I was having a blast. This is probably the one I'd be mostly likely to do, because, even if I started a spinoff instead of resuming with the same characters, a lot of the set-up is already done.
Xarkis or Sarkis or Something
I think it might also be fun to run something in the world of The Nameless Way. I wouldn't try to follow the plot of the novel, because that feels a little too railroady. There are plenty of places to explore that I don't even go in this book, and of course the ones the book does visit still have other adventures to offer.
Ever since Adventures in Middle Earth was announced, I've been turning the thought of a campaign over and over in my head. I've been thinking that, deviating from the official line, I'd run an alternate history where Isildur destroyed the Ring instead of keeping it, but there's still other stuff going on which I'm not going to get into right now. Some of that stuff helps explain why the present-day world (right after the events in The Hobbit) isn't really all that different. A drawback of this one is that I'd have to get the players to acquire and understand their own copies of the Player's Guide. The classes and the way characters are created deviate so much from standard D&D that, otherwise, I would go crazy, constantly trying to explain how everything works.
But of course I probably won't be doing any of these any time soon. Maybe after I catch up on some other stuff. I dunno.