It'd be a medieval type game, most likely a fantasy. Along the lines of elder scrolls. Nothing innovative about that. But here's the catch. Up to a certain point, your character won't die. Can't. Could sneakily code this in, in something like as your health drops, the more damage you deal and the less you take, so it's hard to tell you can't die (obviously the community will figure it out with time, but whatever). Then, in an epic cutscene or battle or something, your character is slain. After that, you take the roles of characters playing some kind of important part to the main storyline (with side quest on the side, of course !

For the tl;dr (i'd recommend the read though

You make a main character, die about halfway through the main story
Take over important/semi-important characters, complete quest (main and side quest) and eventually they die
Eventually take over the 2nd most important character (2nd to your main character) and finish the story. This character doesn't die.
Also talking to a friend about this idea, he gave some cool thoughts.
Have actions being able to be taken by the characters, and that directly affects the story and future generations. IE save a kid, his dad is a general in the imperial army, the general defects and joins you for saving his son
Or have something like at the end of the main character story, eh's possesed and becomes a tyrannical king and you have to destroy him as your son. He keeps your attributes you had at the end of his story, and he plays like you did (done by logging your attacks and how you attack to the DB, then go to a straight ratio) and depending on his level is how many support soldiers he has (to keep the same difficulty, some could tank his low level to be able to beat him easier on multiple playthroughs)
Endless possibilities !