Tradition suggests it would be senior associate Justice John Paul (no relation to the late pope) Stevens. Of course I hope it's Justice Clarence Thomas.
Personally, I would LOVE to see the tradition of having the most senior associate Justice do the swearing in of Chief Justice Roberts.
This would be a major DIG into the conscience of the libs that their power on the USSC is dependant on the daily health and performance of the oldest justice on the court. That would bring into their lives yet another source of worry.
For me, Schedenfreude is a wonderful thing... lol