It is interesting that more games have not used this in re-release of older games. I can think off the top of my head 3 games that I want more content to extend the replay-ability.  
  -Mario-Kart with procedural generated race tracks after you beat all the levels and gain all the trophies.   
  -Multi-Player Halo.  An option for one of the random games to have a procedural generated map.  With a player vote at the end of the game.  Top maps added to the rotation.
  -Burnout paradise- Use the city generator to create random games.  Again players vote at the end of the game.  Top maps added to the rotation.
  Bonus-Left for Dead and make it through the random city.