Ye olde level cap discussion.
      

  Let me follow that up with a disclaimer - We aren't thinking about an expansion at all yet, but as levels are intended to pace content (we expect you to hit the last level around when you kill the last boss on Hell) it's not unreasonable to assume that additional levels would be present in an expansion, assuming it did offer additional content we..
      

http://www.blizzblues.com/us/ye-olde-level-cap-discussion-27805270346.html