The Rock and Undertaker had a feud in mid 1999, but unfortunately it wasn't as big as it could have been for a number of reasons, mainly that it didn't last long and it was clearly second priority on the card behind Austin/McMahon.
Now the Rock is back and the Undertaker will need a quality opponent for Wrestlemania next year. I think a rematch between the two might be an ideal fit for the show.
Both are huge faces, but a feud with the Undertaker might allow that Rock to go back to being a kind of pseudo-heel, sort of the way an otherwise solid face Shawn Michaels did in the run up to his match with Hulk Hogan in 2005.
Discounting their rather average match at the 1999 King of the Ring, these two have never had a top level match against each other on a major PPV.
Wrestlemania XIX just might be the place to change that.