Idea:
A coop class based RPG revolving around a party of fully voice-acted customized characters chosen by the players. Character backgrounds and attitudes could be chosen by players as well with all dialogue revolving around an Alpha Protocol type system where players choose tones for their responses. Setting, overall game story, doesn't matter.
It needs a background story unique to each character, and during cutscenes some characters may be taken away from the rest of the party to experience story elements unique to only them. (Players should be advised to keep these story elements to themselves for the full experience)
As the game progresses, these unique "secret" story elements unique to those character may be revealed to everyone eventually, developed by the decisions made by each of those individual characters. One person may turn out to be a secret bad guy, 2 people may conspire to kill off another person who is then replaced with another party member for him to control. And so on.
THESE ARE JUST MY IDEAS, DON'T HIT ME.