Okay everyone, I just got back from reading the big post credits scene on Wikipedia, and I just have to say I was NOT expecting that.
Spoilers ahead here, but when Dom calls a family meeting in his secret underground car bunker that he's totally had this whole time, he announces that they're adding a new member to their ranks, a sleek and gorgeous red car drives into the room, does a kick-ass doughnut and winks at the audience. Yes, winks. Because this particular red car is none other than LIGHTNING ****-MOTHERING MCQUEEN! Man! I was NOT expecting the giant Furious/Cars crossover, but I think it's pretty obvious that F&F7 with Lightning McQueen is going to blow us all away.