Fumito Ueda (the developer of both ICO and Shadow of the Colossus) has said in the past he won't do sequels. With that said, all of his games seem to be set in the same "universe", but in different times and not necessarily in the right chronological order (ICO was released first, but it seems as if SotC happens before ICO). Check out the coverage of "The Last Guardian" which is the next game that will be released by Team ICO sometime next year.