Back in April SFFT promoted the opening of the 2013 season of their water park with the title "The Regions’ Most eXtreme Water Park Opens Weekends Beginning April 27 at Six Flags Fiesta Texas." They also used the term eXtreme Thrills in the article to promote the water park attractions. My bet is on a new water park slide.
Link to SFFT article: http://www.sixflags.com/fiestaTexas/info/news_2013wwb.aspx