I really like making games, but a lot of stuff I do... well, it doesn't always pan out. The stuff I manage to produce is so-so, but there's a pile of wreckage behind me that just goes to show how much trial and error can go into a project you're passionate about.
But some of my dead ends were funny, too, and I'll bet there's a lot of cool half-baked ideas the "greats" of the story game world may have had.
Or not. But those of us who flop regularly don't mind sharing.
Here's two of mine:
Back before the FORGE even existed, I tried to create a board game based on St. Canard (because I really liked Darkwing Duck.) I drew up a representation of the city and made up some cards and, well, it didn't work. I had tried to make a chase-and-matching game without anything to chase and match or any kind of rewards for the players.
More recently I spent about a month putting together a tabletop RPG that emulated the 8-bit computer RPGs. Oddly enough, it took an 8-year-old to explain to me the obvious reason it wasn't going to fly. (I still thought the bit about having each player having an NPC combat style and stock NPC response along the lines of "They say there's a cave behind the waterfall" for non-PC interactions with other was pretty cool.)