I usually come up with more ideas for what I think will be fun campaigns than I can actually manage to run. Currently, my West Marches game is going great so I've got no desire to change it. And realistically I shouldn't be running any more face to face games. The occasional test of Caverns & Cowboys is the exception to this.
But if I could, I'd be running all kinds of games. I'd need to be independently wealthy to run them all, of course. Maybe when I'm retired this will be a way to spend my time and keep my mind sharp as I get older.
In no particular order, here are ideas I have that I've never gotten to run, or ideas that I attempted but got cut off due to my schedule, lack of player interest, or problems with the execution on my part (which I think I could do better now).
The Maritime Campaign. Back in 2010, I worked up this campaign and started it off. But the players and I had differing expectations of what the campaign was to be about. I set it up as an island hopping treasure-seeking band of heroes with a ship (like Jason and the Argonauts, or Sindbad). One player wanted it to be a mercantile simulation, another wanted it to be a game of piracy and fleet building. Other players were pretty uninterested. It died because I lost interest in figuring out markets for fleece in foreign lands and dropped it.
With the right group of players, I still think it could be a blast.
Terminator: War Against the Machines. This would be a game set in the "future flashbacks" of the Michael Biehn character in the first Terminator (and other films in the series). A short term campaign with a set goal of either to take down Skynet or die trying. Not sure what game system I'd use. d20 Modern/Future could work, but it's a bit clunky in areas. Stars Without Number might be a better fit, except without the psychics. There's also the venerable Star Frontiers, or maybe something generic like the WEG Open d6 system.
Land of 1000 Dragons. This is a fairly new idea I've had. Pretty sure I've mentioned the Terminator idea before, and I posted over 20 times about the Maritime Campaign. This game would probably be best with a modern 'high power PCs' system like 5E, or if run in old school D&D we'd need to start at 5th or 6th level. In Land of 1000 Dragons, there is an open sandbox world, and EVERY lair or dungeon is the home of a dragon. Of course there will be other monsters cohabiting (or lurking and scavenging the leftovers). But the whole game would be focused on dragons. Lots of dragons.
Some sort of Supers game. I'm not sure exactly what the hook should be for this. Supers as a genre is really about preserving the status quo, while most RPGs seem to work best around groups that work outside the system and upset it. If I ever do run a game, it would probably be best to set it in some sort of Sin City type setting, where everyone and everything is corrupt except for our heroes. Or, flip it and run a supervillain game! Let the players run amok until the local heroes get on their tail, then come up with plans to foil the caped crime-fighters! That could be fun.
East Marches. I mentioned this the other day. My West Marches game is going so well. The format makes it really easy for players to buy in, and for me to prep for games. I'd love to run a second game in this format, but with Asian fantasy themes (there are some in my WM game, but it's for the most part standard D&D fantasy). I could easily use the TSR-East classes I'm working up now for this. But I really don't have time to prep and run TWO Marches-style games at the same time.
Fiendish Pits of Dr. Lao. This is an idea I had many years ago. An Asian fantasy themed megadungeon. I'm less keen to run this game these days (and I realize I must have gotten the name Dr. Lao from an old Tony Randall movie -- reading the description I don't remember seeing it before, but I probably heard of it somewhere along the way). I think East Marches would be a better game. But I could probably throw a smaller version of the Fiendish Pits into the East Marches. Or work this idea up as something to try and sell some day.