My first idea is a space-based MMO Flight Sim/FPS/Econ Sim. Spoiler'd for wall of nerdy text.
Obviously accelerating fast enough to reach 50% of the speed of light in less than a year is going to generate enough G-force to kill anyone in the spacecraft. To overcome this i have created a fictional device called the Inertia Controller. With the IC running anyone within the spacecraft experiences a sense that the spacecraft is accelerating straight up at approximately 10m/s2. IC disruptors, similar to EVE's warp scramblers, can damage the field produced by the IC, reducing it's power output. It can still produce artificial gravity, but the spacecraft is limited in acceleration speed.
I mentioned above that this is a flight sim. It doesn't use menu-driven commands like EVE, but instead spacecraft are actually flown by the pilot. Spacecraft are not single objects the way they are in many games, but are instead an enclosed map on which players walk, sit, and operate the spacecraft and its equipment. Not only does this give you something to do on long trips (some objects in this Solar System are over 100 AU from sol, meaning a trip could conceivable be as far as some 200 AU. At the speed of light that trip would take over a day.) but it adds elements of wreck survival, engineering, locational damage, and boarding teams. For example, a missile impact may breach the hull in a certain area. Anyone in that area would be sucked out into the black and die, and the craft's commander must seal that area off or risk a life-support failure.
Unlike EVE, a single person cannot operate an entire spacecraft, at least not a big one. Effectively using all turrets, remote drones, remote missiles, electronic warfare tools, and lord knows what else will require additional personnel to man them. Think ship-to-ship combat in "Pirates of the Carribean Online". Because smaller spacecraft may not have the fuel storage to travel long distances carriers may become popular, too. Also, in order to keep everything working on a large craft engineers (also players) will be needed. They can fix equipment damaged during combat, and optimize equipment for tasks at hand on the fly. They can also help equipment work better than normally possible.
Operators are players who don environmental battle-suits and small arms and force their way onto another spacecraft to take it. They can do this through open hangar bays or by cutting their way through the hull (which might take a while). They will have to battle remote controlled drones, any turrets on the craft that can hit them, and other operators on the victim spacecraft. If successful, however, they will have a free craft. They are able to reach other spacecraft either by jetpacks or by their boarding craft ramming it. They stay attached to the hull with electro-magnetic boots. This is the FPS section i mentioned earlier, though i'm toying with making it a third-person shooter.
The basics of combat and travel having been covered, it's time to get into the economics. There are a lot of asteroids out there. They are mined by landing on them and using mining equipment mounted on the spacecraft or hand carried by the miner. Or both. This creates an opportunity for piracy without even having your own ship, just several operators hiding amongst the asteroids. Players can also land on planets and moons (the map size probably won't be as big as actual planets and moons) to harvest resources or set up processing/production facilities (as compared to space stations, which are also a possibility). The metallic cores in gas giants are also available, though the environment there requires the highest quality environmental suits.
This seems like a good time to break and mention monsters. Obviously if you're on a planet or moon in the goldilocks zone you're going to have to deal with lions, tigers, bears, and xenomorphs. If you're on a planet outside of that you just have the xenomorphs to contend with. Dangerous atmospheric beasts will occupy gas giants, and even in space monsters can be found floating about Zerg style. In addition to simply attacking players (especially miners) the young may crawl into spacecraft and hide out in maintenance shafts, potentially resulting in the first "Alien" movie.
I have utterly failed to think of a skill system that i think would complement this well, and am thinking about not having one. You can use anything you can afford to buy, basically.
Idea the second: comedy RPG
Pancake Quest: The Most Important Meal of the Day
After this we are given control of our hero and directions to get him dressed and march him to the general store for more pancake mix. As you walk down the street you are periodically passed by patrol cars with their sirens screaming and knights on horseback, also with sirens. After a short ways you pass them again, now fleeing in the other direction. When you round a corner you see the general store just in time to see a man with cartoon-style TNT strapped to him. He immediately detonates it, taking the general store with him. Our hero rushes forward, hoping to find some pancake mix that survived the blast. Instead he finds a man who had been facing sideways from the explosion so that one half of his body is severely burned and the other is normal. Our hero digs a bandaid up from the wreckage and applies it to his new friend's cheek. The man somberly thanks our hero and pledges vengeance upon the people responsible for attacking the general store, asking if he may follow our hero on his quest. Also being exploded has given the guy powers over fire for some reason. Our hero cares nothing for this man's vengeance quest, but he doesn't want to go home and he doesn't know how to get to the grocery store across town, so he agrees to help him.
That's as much of the story as my friend and i worked out specifically. You can beat the game by going straight to the grocery store across town, however you will meet many people along the way and have many opportunities for side quests which will involve organized crime, political intrigue, totalitarian city governments, and corporate espionage. Our hero will play a pivotal role in all of these things, and will give a shit about none of it. He never voices his apathy, however, so he is lauded as a hero by the end of the game when in reality he only wants pancakes. Two plot points we have worked out is that at the end of the bomb victim's side quest he will be faced with the guy who ordered the suicide bombing of the general store. He will then demand that the man "look at the damage he has caused!" at which point he will yank off the bandaid on his cheak, revealing charred skin that looks just like all of the other charred skin clearly visible on his body. The villain will be shocked at this sight, but will still fight to defend himself. Also, at the end of the game, after you have defeated whoever the final boss will be and gotten to the grocery store to get pancake mix. The closing cinematic will show the hero arriving home and starting the pancake process again. He pours out two pancakes in a skillet when his phone rings. What proceeds is a few minutes of akward conversation with his mother. After he finally gets her off the phone he returns to his pancakes and flips them to show that they burned while he was on the phone. Our hero casts his spatula to the ground, raises his arms to the air, and screams "NOOOOOOO" as the screen fades to black, leaving us open for a sequel.
As for the actual mechanics of the game, we're thinking something similar to the open system they used in FF12, except not retarded. Each character will have weapon and armor styles they specialize in (we want to introduce a hippy anarchist chick who fights with molotov cocktails and a gun-nut who fights with various home firearms. Virtually all of the characters will be some manner of caricature) as well as various skills. Haven't decided on a skill system yet. One thing we definitely want, however, is what we call the e"quip"ment system. In this game the characters will launch various quips at friends, enemies, and eachother during play. For example, cutting off someone's head and saying "Not a good time to lose one's head!" Basically the kind of thing you'd hear in a Bond movie. The characters get access to new quips depending on what equipment they're wearing. Cooler looking equipment, like fedoras and heel-boots, will allow for more and better quips, but are less functional as armor and weapons.
Those are my retarded ideas, what do you think? And what are yours?