I think them imploding will be one of the main storylines for the summer. Ryback backing out from the Wrestlemania match (we presume, anyway) means they will likely win and then continue their dominance. From there on, there's interesting possibilities like the ones mentioned (like adding one or two other NXT members to the group, although that might be a risk because of the three we have seeming so perfect and there might be a risk in the balance of the group by adding new members) or them attacking CM Punk for whatever reason. Punk being the victim of one of their attacks would indeed be something, as it would show they play no favorites at all and will actually strike at someone they once aided.
I personally see the beginning of their breakout being when they first get owned by various members of the WWE locker room. People start to strike back on The Shield in large numbers and take them out one by one. They put one member on the shelf (Rollins), and that leaves only two left to do the attacking over the next couple of weeks or so. Then another member goes down (Ambrose) and that leaves one guy left. I leave Roman Reigns last because he fits this last idea the best being the biggest of the group and stuff. Realizing he's all alone, he comes to the ring and screams into the mic for all who want any of him to come and get him (this would be similar to a person's final war cry when they realize their time is up and they want to go out in a blaze of glory) and then people run out to get Reigns. He fights them off but they eventually put him down and smash him through tables and stuff. It looks like a big victory for the WWE locker room and makes The Shield look strong because it shows the three of them together were so powerful that they had to be dealt with through gang attacks as well.
From here on, the group (when it does return) has a bit of friction between them that might lead to their break up. As others predict, Rollins will likely be the breakout baby face of the group, and I can even see Ambrose and Reigns just being on their own (just the two of them) for awhile. Whatever happens afterwards though, I see various gang attacks on The Shield as being their ultimate karma.