Well, like it was previously said earlier. Shawn Michaels isn't going to be around much longer. He is going to get surgery after Summerslam or after the next PPV.
DX was just to help ratings for the summer, and to help Triple H turn face.
Plus Shawn isn't to happy about being in DX anyways because of his new Christian ways. He already complained to McMahon about the angle.
BUT.
If they do decide to bring in another person. It will definently be after Shawn leaves. It won't be Cena, for the mere fact that he already has established himself as a wrestler. They won't bring back Roaddog or Billie Gunn because they are in TNA. They won't even think about bringing in X-Pac. He's been on and off drugs ever since WWE. Same with Jeff Hardy. Chyna is more then likely doing some kind of porn with X-Pac again. Matt Hardy would be the best bet, but they probably won't do it.
So, after Summerslam or the next PPV, don't expect DX to be around.