Excellent question! I spent a lot of today pondering my options on that. I've had one particular idea marinating in the juices of my brain for quite a while, and only today did I realize that it's really much too big and complex to be done quickly (that's usually something I have trouble detecting...). Today I was bumbling around imagining what a cut-down version would be like, but I just am not sure. It's all very confusing.
Idea #1: Questies! (exclamation mark required) This is the one that is too big. It's an RPG that is loosely based on Costume Party, stylistically. The rough idea is that of course you make your own quests/dungeons/adventures (not 3 separate things, just covering all my bases), and players go adventure in them. It plays like WoW in the sense that you click on different skills to use them, but like Costume Party in the sense that you hop from place to place on platforms in a side view (easy platforming, unlike Costume Party, no tricks or traps). The leveling is a cool system I came up with looong ago (I just checked - the files that first discussed it are from 1999!) and have always wanted to use. And another key component, from that idea, is that you create many characters and build a town with them and they work together. Literally in fact - you have a team of 3 guys adventuring at a time. One you control, and the other two function automatically, doing their backup thing depending on what class they are.
Idea #2 - MMOTD. It's a tower defense game. Build your towers, and await the invasion. What is interesting about it, besides the fact that the attacks don't follow a path and you need to defend all sides (and can buy scout towers to get advance warning of where attacks will come from), and can have soldiers that run out and attack as well as towers, is that attacks come at midnight each (actual) night. You then get to watch a playback of the events whenever you log in. If you miss a day of play, you'll have been attacked twice since your last visit, and you could eventually lose. There could also be a pause option - no attacks, but no income or experience either. It's one of those games where you build up your city, getting some money each (real life) day to work with, but what you are building is your TD defenses. Then of course for the fun of it, you can capture monsters and unleash them on other peoples' towns (probably need a PVP on/off option) when they get attacked that night. When you lose, you can start over in a new town, but your character (a military strategist who advised the late king of your previous town) has learned more powerful buildings and such from the experience he gained in the first town. When you are strong enough, you can elect to start your new town in a more dangerous area. Every area gets tougher each night, so you will die eventually, but your character will continue to grow (oh, and also he steals some of the king's money when he leaves, so you start each new town with some money left from your last town). I've had this idea rattling around my noggin for a couple of years as well, but not nearly as long as the other.
So those are two rather big ideas I've been mulling. I should probably try something truly small and simple instead, not overreach. Space Invaders!