The one I regretted the most was 'Universe City' a Gamma World campaign idea. The concept was that, when the apocalypse happened and huge cyberpunk-archology-style city-buildings were being vaporized, there was a group of re-enactors /under/ one of them, in the historical site of a 20th century college town that vast structure had been built over, complete with an artificial sun/moon/stars on the duralloy dome covering it. With everything outside their re-enactment radioactive slag, they went a little nuts, and founded a culture based on their romantic view of the 20th century, hiding away all knowledge of later history and technology, and imposing their fanaticism on the few unfortunate tourists visiting them at the time. By the time the radiation died down and mutants form the rest of Gamma World started digging their way in, they'd become this mad, insular society, with several dogmatic factions vying for power...
...thing is, I finally did get to run it 20 years after I had the idea, 3 editions or so of Gamma World later.
One that I never got to do was a Tragic Millennium campaign, 1e AD&D heavily modded to handle the setting of Moorcock's Runestaff series.
Another was The Seven Cities, inspired by 3e, but would have been perfect for the 4e 'points of light' theme, the idea was that there was huge continent of deadly (really, insanely, high-level characters routinely die deadly) wilderness (The Wilds) with seven walled-off cities (long ago part of a single civilization, now connected only by underground tunnels) scattered across it. The underdark was actually the less dangerous adventuring area, with the cities the safest, and the wildernesses separating them the 'high level' adventuring areas with the deadliest threats and greatest treasures (in ruins of other cities and settlements that didn't survive). The Wilds weren't just deadly dangerous, but actively hostile to sentient life, with some sort of force behind them. Druids were particularly creepy, since they had some understanding/influence of The Wilds - they also tended to be driven mad by that connection - while Necromancers were valued, contributing members of society (skeletons and zombies did a lot of the menial work).
But the classic theme of 'man vs nature' just doesn't go over with modern audiences.
