The game itself is basically an elder scrolls or fallout 3 type game, but I have an idea for an interesting concept in those types of games. 
it's a mix of single and mulitplayer but you never actually meet another player. like dragon age you start by picking your origin which determines where in the world you started and what quests you can do etc etc. 
 
Here is the interesting part after you pick your origin your game is linked with people that took the other origins. you don't play with them it's just your game reading whats happen is that person's game and their game reading whats going on in your game.  
its like a mmo where the quests have meaning there is not a line of 100 people behind you and ahead of you doing the same quest 
its a dynamic quest system where evil guy enslaved town but you freed town so back in evil guy's game the town is freed so he can come back and kill everyone just to get back at you or evil guy goes back to that town and gets jumped by the people he enslaved.
 
This is basically the big example. You come to some small town and it's enslaved because one of the players in their game had a quest where they choose to be evil.
Then you can have a quest to free the people and the quest will name the player that did it by name, which is awesome if smokedog 420 was the player's name.
If no one that your linked to got to that town you'd get the quest and stuff like that. 
 
I just thought up another example you did a quest to get some guys weapons back. in another person's game they have quest to rob that guy.
the robber runs into a armed guy now. if you didn't do that quest the the other player would have it easier because the guy would not be armed
 
The problem is you need thousands of quests because then one guy would just do all the quests and leave you with nothing to do.
Its a really rough idea that I was just thinking about, tell if you think its stupid or interesting I'm not saying this is an amazing idea so don't get to pissed at me.