The Egosoft Website (thanks Frans) has details on how the X2: The Return, their space combat expansion, has become X3: Reunion, a full-fledged sequel:

EGOSOFT today announces the beginning of a new era for the X series of space combat/trading games. The planned expansion for the award winning PC CDROM title, X²: THE THREAT known as X²: THE RETURN has been cancelled. The essence of the expansion pack has been used to give birth to the next generation of X game, in the form of X³: REUNION, the first multi format product, set in the X universe.

During the creation of X²: THE RETURN, it became apparent to EGOSOFT that the expansion pack had taken on a life of its own. With so many ideas from the fan community, waiting to be implemented into an already crowded expansion game and development of the X³ engine moving at a rapid pace behind the scenes, the conception of a full, stand alone game became the logical choice.

X³: REUNION utilises a new generation game engine, developed to immerse the player into the most realistic, living universe, ever experienced. Extensive development has gone into the X³ engine, making full use of DirectX 9 technology, to create dramatic visual effects and stunningly realistic starships. The economy model in X³: REUNION is more complex than any previous X game. Coupled with the massively enhanced A.L. (Artificial Life) system, X³: REUNION will present players with an ever changing, evolving universe; where a player’s actions really can shape the future of the universe. Community involvement has been extensive, throughout the development process and the launch of X³: REUNION will see many innovative fan creations in the final retail game. X³: REUNION will also be the first chance for players to experience the thrills of deep space trading on a format other than PC, with the launch of a console version (TBA).