RPG Blog Carnival this month, especially since the topic is so easy: Campaigns I'd Like to Run, a topic chosen by Lowell at Age of Ravens.
Ironically enough, one campaign I've wanted to run for a while--a supers game--I'm doing right now, so I can scratch that one off the list. So what have I dreamed of doing? Two answers
My Own Version of the West Marches
I think most GM's of fantasy games have considered this at least once in their lives. For those who don't know, the West Marches was a D&D sandbox campaign run years ago by Ben Robbins over at Ars Ludi. His reflections on it were immediately absorbed by a lot of OSR people into their canon about what a sandbox campaign should look like. What's ironic was that a) it wasn't a hexcrawl, and b) it was done using the oft-maligned 3rd Edition rules.
In any case, I've often dreamed of doing a fantasy campaign where the focus was on player's deciding what they wanted to do, a lot of exploration over monster-exterminating, and a structure where players could drop in and out as their schedules dictated. As an added bonus, I would want it to be a good "gateway" game for people new to the hobby. For rules, I could practically any fantasy RPG. Mi Gran Sueno.
Think futuristic Roanoke colony. A group of space explorers end up on a planet far from anything. Basically, they will never, ever experience contact with their home civilization again. The planet is mysterious and hostile. So basically every bullet matters, every NPC matters, every resource matters. Still an exploration and sandbox campaign, but with a bit more tension. Plus since it is on an alien planet, there's no way for the players to be able to identify in that "I've read the Monster Manual" way any of the plant or animal (or sentient) life they come across.
For mechanics, I've always had a soft spot for d6 Space, although Traveller would do in a pinch.