I think it would make no sense if Ryback joined the shield and if the reasoning is to get back at Cena, then it would just diminish his juggernaut character. But if they pulled something like everything that the shield did to Ryback was all smoke and mirrors and the plan from the get go was to destroy Cena and Ryback has always been the leader, then that could work. Then again the smarks will come out in droves saying that the WWE copied Bully Ray's storyline, too soon to be a coincidence.
But that still wouldn't explain the HITC Maddox debacle, unless they play it off as a CM Punk Heyman rouge move. But once again they had a program of the shield being in on it and jumping Maddox.
It would take some serious storyline writing to explain the past 6 months. Unless the WWE thinks we all went senile.
I also think if Ryback joined the shield he would have to tweak his character to be more cocky on the mic, unless he just plays a hard breathing monster in the background of shield promos.