There's no "official" sequel but Tolkien has notes regarding events that WILL happen after RotK like Dagor Dagorath which is a sort of Ragnorak or final battle for the LotR universe.
http://lotr.wikia.com/wiki/Dagor_Dagorath
He also wrote some stories about a cult of Sauron or Morgoth that takes place in Gondor somewhere in his notes. When I was 14 I actually wrote a fanfiction based on it which fleshed out the main character as this warrior from the "Tribe of Morgoth" who seeks to infiltrate Minas Tirith but ends up falling in love with one of Elessar's descendants.