Imagine, if you will, a different version of the Mass Effect series. One where everything pretty much happens the same way, all the features are still the same and all the options you had at your disposal were still available. Except, instead of a game based around the choices you make, this is a game based around the choices you and your sibling make.
A game where one player may have chose not to shove that smart mouthed merc out the window, but as he or she turned their back, the other kicked the poor merc right through the glass. When Kai Leng was about to sneak up on you, your partner reacts first and gets the killing blow. A game where some crew members are closer to and fight better with one Shepard while others align themselves with the sibling based on how the players acted and treated them. A game where instead of one squad of three people taking on ridiculous tasks during combat, you now have a partner and his or her own squad to back you up. Maybe by the end of the series, you and your partner differ on which color you want to paint the galaxy and you have to do the unthinkable to make sure him or her doesn't get their way.
I want to play that game. Just to fuck with my friends and steal their waifus.