While it's true that I'm working on a rather elaborate game right now, there's also a few I wish I could make but for one reason or another, I decided not to.
The Toonamu game, where it was like, a multi-theme kind of deal, primarily centered around anime. I mainly didn't do it because I realized that I didn't really enjoy mashing together so many things that didn't quite fit seamlessly, and I had trouble envisioning where I would take the game if it did get finished.
A Power Rangers MUSH is another one I always wanted to do. My main reason for not doing it is mostly because it's such a niche thing that I don't think most of my friends would play or be interested in playing. I'm sure plenty of people would, but when I make a game I really like it to be something that all of my friends will play, it's just how I am.
I also always wanted an SCP game, but I just feel like I'd run out of steam for running it quickly, I don't know. It was always kind of a shaky foundation (ha ha) to build a game on top of.
And lastly, I'd say, a big one I always wanted probably more than these others. An L&L game based entirely in Hell, where people are denizens of Hell doing L&L shit. I think a lot of people would enjoy and play that, and it lacks most of the problems of these other concepts. But my main reason for not making it is mainly all the OOC drama that comes with running an L&L game. Though I have considered secretly building this MUSH and paying for the server space, but giving it to @Jealousy to run, mostly because I think it would be hilarious to see her run a MUSH.