I was going to ask why you didn't post this in the Game Design board, and then I remembered it is a ghost town.
I have a couple of dormant ideas. Most of the ideas I have are adding unique twists to things like text-based MMOs, mainly because I've played a few and found them all to be lacklustre; they never seem to use the player-base to their advantage. This approach has lead me to think about what I would like to see players do in a game, and then design mechanics around that.
Crime/Politics/Economics Text-Based MMO - The idea behind the game is that there is an automated economy, which the players must exploit for their own gain. How they do this is up to them: they might form an organized crime family and terrorize the citizens. Other groups might form political parties to try and gain power and control over the city. Others might start a business to gain power through commerce. I wanted to create a game with conflicting styles of play because I thought it might have some interesting consequences.
Wild West Text-Based MMO - This one was about bringing people together in interesting ways. When playing Text MMOs I've always felt disconnected from everyone else, there's barely any interaction with others at all.
I like the idea of throwing people into random, sometimes dangerous, situations and seeing how they react; for example, if they were travelling across the plains and saw a stranger (another player) being attacked by a cougar would they help, wait to see if the cougar kills them then loot the body, or ignore the situation and keep moving? The wild west seemed like a perfect setting for this type of game. I was also influenced by Red Dead Redemption's multi-player: there's a great tension when you meet other players, it's not easy to kill in that game so you are reluctant to pull your gun and start a fight that would be a waste of time, but you never know what the other guy is going to do. A lot of the time you slowly circle each other a bit until you both come to some unspoken truce and move on. I wanted that kind of tension in my game. I think I was actually talking with Jerico about making this but we both got distracted by other things.
Space Adventure - The concept for this game is all about exploration and dynamic levels of detail. I thought about how games like Skyrim have these huge worlds with smatterings of story and detail scattered here and there. That type of world seems very unauthentic to me, it's like you are walking into different scenes of a play rather than exploring a living world.
I wanted to make a world that built itself around the player.
For example, there might be a rebellion taking place on a distant planet, you decide to travel there to aid the rebels. Once you reach the planet, you walk down streets that had been generated for your arrival and into a bar where people, who until now had not existed, are seated in the corner speaking in hushed voices. You think they are rebels but you cannot be sure, you must find a way to get talking to them without raising suspicion. One of them notices you and whispers something to the others. You think they must know why you are here, but what if they are government spies!? You start to think this whole adventure was a big mistake.
Suddenly a voice beside you, the voice of a large and frightfully ugly creature whom you somehow did not notice before, booms out a string of words in a language you've never heard with such power it overcomes the rabble of the crowded room, "Eeksca Noga Nucasi Booraina Pochee". You are simultaneously shocked, confused, repulsed and scared but fail to hide any of these emotions. You look around nervously for someone to help you but the only eye you meet is that of an elderly local, who squints at you then quickly loses interest once he fails to recognise your face. The only other people in the room who are paying any attention to the situation are a group of youngsters who are now rapidly cluing each other in and snickering at your predicament.
Finally, the head of grey-skinned man (or at least you think it's a man) appears from the other side of the giant, despite appearing to be quite slender and tall he is clearly straining to be seen past the considerable girth of his enormous companion. He peers around the giant with a suspicious expression, "My friend wants to know what a Nucosan is doing all the way out here", you stutter and then say the first thing that comes into your head,
"My ship was hit by some space debris and I needed to land for some repairs."
"I don't think you heard me right kid", the grey man's voice turned deeper and his stare grew sharper, "my friend here doesn't like Nucosans, and neither do I. During the war we were shooting down twenty nuc-nucs a day."
After hearing this line your skin immediately turned pale, your heart beat so violently it felt like it was trying to flee while the rest of your body stood there frozen in silence. The giant rose up towering above you, it let out a fearsome roar, grabbed you by the collar and dragged you out of the bar as you squirmed and kicked in terror. No one dared intervene, not even the mysterious group at the corner table. If they really were the rebels you'd been looking for this was not the first impression you wished to give. Why where you even thinking about that now!? You're about to be pulverized by a giant and all you can think about is how this makes you look? The beast drags you down a neighbouring alleyway and drops you by a wall. The grey man appears, he towers over you from almost the same height as the giant, "Sorry about that mate, but we had to do something, you were sticking out like a sore toe in there. Now, you're not here for repairs are you? Anyone needing repairs would have stopped at Actan, not here."
Were these freaks the rebels?...
...err yeah, like that. Got a bit carried away there.

I would be amazed if it was anything like as detailed as that but the idea is that there are a number of events happening in the universe and the more you look into them the more detail is generated. Unique stories develop procedurally through different combinations of events. It's like the entire universe is a big story tree that you can join and exit at almost any point, but first you have to find a way in. Similar events will take place all over the galaxy but who you speak to and what you do determines the path you take through it. So, basically what I'm trying to do is turn the traditional story arc on its head: instead of the player following the story, the story follows the player. It sounds like magic but I don't think so, all I'm doing is chopping up narrative into tiny pieces and letting the player influence where each piece goes. This gives the player the freedom to go where they want, do what they want, speak to who they want, and the game will adapt to those choices.
The game will be 2D with PG art for ships, planets, scenery and aliens. All the speech will be text-only. I'm not even going to attempt animations.